If the container is not the body, CSS Tricks covers all of your options in Fitting Text to a Container.

If the container is the body, what you are looking for is Viewport-percentage lengths:

The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly. However, when the value of overflow on the root element is auto, any scroll bars are assumed not to exist.

The values are:

vw (% of the viewport width)

vh (% of the viewport height)

vi (1% of the viewport size in the direction of the root element's inline axis)

vb (1% of the viewport size in the direction of the root element's block axis)

vmin (the smaller of vw or vh)

vmax (the larger or vw or vh)

1 v* is equal to 1% of the initial containing block.

Using it looks like this:

p {
    font-size: 4vw;
}

As you can see, when the viewport width increases, so do the font-size, without needing to use media queries.

These values are a sizing unit, just like px or em, so they can be used to size other elements as well, such as width, margin, or padding.

Browser support is pretty good, but you'll likely need a fallback, such as:

p {
    font-size: 16px;
    font-size: 4vw;
}

Check out the support statistics: http://caniuse.com/#feat=viewport-units.

Also, check out CSS-Tricks for a broader look: Viewport Sized Typography

Here's a nice article about setting minimum/maximum sizes and exercising a bit more control over the sizes: Precise control over responsive typography

And here's an article about setting your size using calc() so that the text fills the viewport: http://codepen.io/CrocoDillon/pen/fBJxu

Also, please view this article, which uses a technique dubbed 'molten leading' to adjust the line-height as well. Molten Leading in CSS

---

But what if the container is not the viewport (body)?

This question is asked in a comment by Alex under the accepted answer.

That fact does not mean vw cannot be used to some extent to size for that container. Now to see any variation at all one has to be assuming that the container in some way is flexible in size. Whether through a direct percentage width or through being 100% minus margins. The point becomes "moot" if the container is always set to, let's say, 200px wide--then just set a font-size that works for that width.

Example 1

With a flexible width container, however, it must be realized that in some way the container is still being sized off the viewport. As such, it is a matter of adjusting a vw setting based off that percentage size difference to the viewport, which means taking into account the sizing of parent wrappers. Take this example:

div {
    width: 50%;
    border: 1px solid black;
    margin: 20px;
    font-size: 16px;
    /* 100 = viewport width, as 1vw = 1/100th of that
       So if the container is 50% of viewport (as here)
       then factor that into how you want it to size.
       Let's say you like 5vw if it were the whole width,
       then for this container, size it at 2.5vw (5 * .5 [i.e. 50%])
    */
    font-size: 2.5vw;
}

Assuming here the div is a child of the body, it is 50% of that 100% width, which is the viewport size in this basic case. Basically, you want to set a vw that is going to look good to you. As you can see in my comment in the above CSS content, you can "think" through that mathematically with respect to the full viewport size, but you don't need to do that. The text is going to "flex" with the container because the container is flexing with the viewport resizing. UPDATE: here's an example of two differently sized containers.

Example 2

You can help ensure viewport sizing by forcing the calculation based off that. Consider this example:

html {width: 100%;} /* Force 'html' to be viewport width */
body {width: 150%; } /* Overflow the body */

div {
    width: 50%;
    border: 1px solid black;
    margin: 20px;
    font-size: 16px;
    /* 100 = viewport width, as 1vw = 1/100th of that
       Here, the body is 150% of viewport, but the container is 50%
       of viewport, so both parents factor  into how you want it to size.
       Let's say you like 5vw if it were the whole width,
       then for this container, size it at 3.75vw
       (5 * 1.5 [i.e. 150%]) * .5 [i.e. 50%]
    */
    font-size: 3.75vw;
}

The sizing is still based off viewport, but is in essence set up based off the container size itself.

Should Size of the Container Change Dynamically...

If the sizing of the container element ended up changing dynamically its percentage relationship either via @media breakpoints or via JavaScript, then whatever the base "target" was would need recalculation to maintain the same "relationship" for text sizing.

Take example #1 above. If the div was switched to 25% width by either @media or JavaScript, then at the same time, the font-size would need to adjust in either the media query or by JavaScript to the new calculation of 5vw * .25 = 1.25. This would put the text size at the same size it would have been had the "width" of the original 50% container been reduced by half from viewport sizing, but has now been reduced due to a change in its own percentage calculation.

A Challenge

With the CSS3 calc() function in use, it would become difficult to adjust dynamically, as that function does not work for font-size purposes at this time. So you could not do a pure CSS 3 adjustment if your width is changing on calc(). Of course, a minor adjustment of width for margins may not be enough to warrant any change in font-size, so it may not matter.

---

Solution with SVG:

.resizeme { resize: both; margin: 0; padding: 0; height: 75px; width: 500px; background-color: lightblue; overflow: hidden; }

Solution with SVG and text-wrapping using foreignObject:

.resizeme { resize: both; margin: 0; padding: 0; height: 200px; width: 500px; background-color: lightblue; overflow: hidden; }

---

In one of my projects I use a "mixture" between vw and vh to adjust the font size to my needs, for example:

font-size: calc(3vw + 3vh);

I know this doesn't answer the OP's question, but maybe it can be a solution to anyone else.

---

Pure-CSS solution with calc(), CSS units and math

This is precisely not what OP asks, but may make someone's day. This answer is not spoon-feedingly easy and needs some researching on the developer end.

I came finally to get a pure-CSS solution for this using calc() with different units. You will need some basic mathematical understanding of formulas to work out your expression for calc().

When I worked this out, I had to get a full-page-width responsive header with some padding few parents up in DOM. I'll use my values here, replace them with your own.

To mathematics

You will need:

Nicely adjusted ratio in some viewport. I used 320 pixels, thus I got 24 pixels high and 224 pixels wide, so the ratio is 9.333... or 28 / 3

The container width, I had padding: 3em and full width so this got to 100wv - 2 * 3em

X is the width of container, so replace it with your own expression or adjust the value to get full-page text. R is the ratio you will have. You can get it by adjusting the values in some viewport, inspecting element width and height and replacing them with your own values. Also, it is width / heigth ;)

x = 100vw - 2 * 3em = 100vw - 6em
r = 224px/24px = 9.333... = 28 / 3

y = x / r
  = (100vw - 6em) / (28 / 3)
  = (100vw - 6em) * 3 / 28
  = (300vw - 18em) / 28
  = (75vw - 4.5rem) / 7

And bang! It worked! I wrote

font-size: calc((75vw - 4.5rem) / 7)

to my header and it adjusted nicely in every viewport.

But how does it work?

We need some constants up here. 100vw means the full width of viewport, and my goal was to establish full-width header with some padding.

The ratio. Getting a width and height in one viewport got me a ratio to play with, and with ratio I know what the height should be in other viewport width. Calculating them with hand would take plenty of time and at least take lots of bandwidth, so it's not a good answer.

Conclusion

I wonder why no-one has figured this out and some people are even telling that this would be impossible to tinker with CSS. I don't like to use JavaScript in adjusting elements, so I don't accept JavaScript (and forget about jQuery) answers without digging more. All in all, it's good that this got figured out and this is one step to pure-CSS implementations in website design.

I apologize of any unusual convention in my text, I'm not native speaker in English and am also quite new to writing Stack Overflow answers.

It should also be noted that we have evil scrollbars in some browsers. For example, when using Firefox I noticed that 100vw means the full width of viewport, extending under scrollbar (where content cannot expand!), so the fullwidth text has to be margined carefully and preferably get tested with many browsers and devices.

---

There is a big philosophy for this issue.

The easiest thing to do would be to give a certain font-size to body (I recommend 10), and then all the other element would have their font in em or rem. I'll give you an example to understand those units. Em is always relative to its parent:

body{font-size: 10px;}
.menu{font-size: 2em;} /* That means 2*10 pixels  = 20 pixels */
.menu li{font-size: 1.5em;} /* That means 1.5*20 pixels = 30 pixels */

Rem is always relative to body:

body{font-size: 10px;}
.menu{font-size: 2rem;} /* That means 2*10 pixels = 20 pixels */
.menu li{font-size: 1.5rem;} /* that means 1.5*10 pixels = 15 pixels */

And then you could create a script that would modify font-size relative to your container width. But this isn't what I would recommend. Because in a 900 pixels width container for example you would have a p element with a 12 pixels font-size let's say. And on your idea that would become an 300 pixels wide container at 4 pixels font-size. There has to be a lower limit.

Other solutions would be with media queries, so that you could set font for different widths.

But the solutions that I would recommend is to use a JavaScript library that helps you with that. And fittext.js that I found so far.

---

Here is the function:

document.body.setScaledFont = function(f) {
  var s = this.offsetWidth, fs = s * f;
  this.style.fontSize = fs + '%';
  return this
};

Then convert all your documents child element font sizes to em's or %.

Then add something like this to your code to set the base font size.

document.body.setScaledFont(0.35);
window.onresize = function() {
    document.body.setScaledFont(0.35);
}

http://jsfiddle.net/0tpvccjt/

---

There is a way to do this without JavaScript!

You can use an inline SVG image. You can use CSS on an SVG if it is inline. You have to remember that using this method means your SVG image will respond to its container size.

Try using the following solution...

HTML

<div>
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360.96 358.98" >
      <text>SAVE $500</text>
  </svg>
</div>

CSS

div {
  width: 50%; /* Set your container width */
  height: 50%; /* Set your container height */

}

svg {
  width: 100%;
  height: auto;

}

text {
  transform: translate(40px, 202px);
  font-size: 62px;
  fill: #000;
}

Example: https://jsfiddle.net/k8L4xLLa/32/

Want something more flashy?

SVG images also allow you to do cool stuff with shapes and junk. Check out this great use case for scalable text...

https://jsfiddle.net/k8L4xLLa/14/

---

This may not be super practical, but if you want a font to be a direct function of the parent, without having any JavaScript that listens/loops (interval) to read the size of the div/page, there is a way to do it. Iframes.

Anything within the iframe will consider the size of the iframe as the size of the viewport. So the trick is to just make an iframe whose width is the maximum width you want your text to be, and whose height is equal to the maximum height * the particular text's aspect ratio.

Setting aside the limitation that viewport units can't also come along side parent units for text (as in, having the % size behave like everyone else), viewport units do provide a very powerful tool: being able to get the minimum/maximum dimension. You can't do that anywhere else - you can't say...make the height of this div be the width of the parent * something.

That being said, the trick is to use vmin, and to set the iframe size so that [fraction] * total height is a good font size when the height is the limiting dimension, and [fraction] * total width when the width is the limiting dimension. This is why the height has to be a product of the width and the aspect ratio.

For my particular example, you have

.main iframe{
  position: absolute;
  top: 50%;
  left: 50%;
  width: 100%;
  height: calc(3.5 * 100%);
  background: rgba(0, 0, 0, 0);
  border-style: none;
  transform: translate3d(-50%, -50%, 0);
}

The small annoyance with this method is that you have to manually set the CSS of the iframe. If you attach the whole CSS file, that would take up a lot of bandwidth for many text areas. So, what I do is attach the rule that I want directly from my CSS.

var rule = document.styleSheets[1].rules[4];
var iDoc = document.querySelector('iframe').contentDocument;
iDoc.styleSheets[0].insertRule(rule.cssText);

You can write small function that gets the CSS rule / all CSS rules that would affect the text area.

I cannot think of another way to do it without having some cycling/listening JavaScript. The real solution would be for browsers to provide a way to scale text as a function of the parent container and to also provide the same vmin/vmax type functionality.

JSFiddle: https://jsfiddle.net/0jr7rrgm/3/ (click once to lock the red square to the mouse, and click again to release)

Most of the JavaScript in the fiddle is just my custom click-drag function.

---

Using vw, em & co. works for sure, but IMO it always needs a human's touch for fine-tuning.

Here's a script I just wrote based on @tnt-rox' answer that tries to automatize that human's touch:

$('#controller').click(function(){ $('h2').each(function(){ var $el = $(this), max = $el.get(0), el = null ; max = max ? max.offsetWidth : 320 ; $el.css({ 'font-size': '1em', 'display': 'inline', }); el = $el.get(0); el.get_float = function(){ var fs = 0 ; if (this.style && this.style.fontSize) { fs = parseFloat(this.style.fontSize.replace(/([\d\.]+)em/g, '$1')); } return fs; }; el.bigger = function(){ this.style.fontSize = (this.get_float() + 0.1) + 'em'; }; while (el.offsetWidth < max) { el.bigger(); } // Finishing touch. $el.css({ 'font-size': ((el.get_float() -0.1) +'em'), 'line-height': 'normal', 'display': '', }); }); // end of (each) }); // end of (font scaling test) div { width: 50%; background-color: tomato; font-family: 'Arial'; } h2 { white-space: nowrap; } h2:nth-child(2) { font-style: italic; }

It basically reduces the font-size to 1em and then starts incrementing by 0.1 until it reaches maximum width.

JSFiddle

---

Artistically, if you need to fit two or more lines of text within the same width regardless of their character count then you have nice options.

It's best to find a dynamical solution so whatever text is entered we end up with a nice display.

Let's see how we may approach.

var els = document.querySelectorAll(".divtext"), refWidth = els[0].clientWidth, refFontSize = parseFloat(window.getComputedStyle(els[0],null) .getPropertyValue("font-size")); els.forEach((el,i) => el.style.fontSize = refFontSize * refWidth / els[i].clientWidth + "px") #container { display: inline-block; background-color: black; padding: 0.6vw 1.2vw; } .divtext { display: table; color: white; font-family: impact; font-size: 4.5vw; }

All we do is to get the width (els[0].clientWidth) and the font size (parseFloat(window.getComputedStyle(els[0],null).getPropertyValue("font-size"))) of the first line as a reference and then just calculate the subsequent lines font size accordingly.

---

Use CSS Variables

No one has mentioned CSS variables yet, and this approach worked best for me, so:

Let's say you've got a column on your page that is 100% of the width of a mobile user's screen, but has a max-width of 800px, so on desktop there's some space on either side of the column. Put this at the top of your page:

<script> document.documentElement.style.setProperty('--column-width', Math.min(window.innerWidth, 800)+'px'); </script>

And now you can use that variable (instead of the built-in vw unit) to set the size of your font. E.g.

p {
  font-size: calc( var(--column-width) / 100 );
}

It's not a pure CSS approach, but it's pretty close.

---

100% is relative to the base font size, which, if you haven't set it, would be the browser's user-agent default.

To get the effect you're after, I would use a piece of JavaScript code to adjust the base font size relative to the window dimensions.

---

I've prepared a simple scale function using CSS transform instead of font-size. You can use it inside of any container, you don't have to set media queries, etc. :)

Blog post: Full width CSS & JS scalable header

The code:

function scaleHeader() {
  var scalable = document.querySelectorAll('.scale--js');
  var margin = 10;
  for (var i = 0; i < scalable.length; i++) {
    var scalableContainer = scalable[i].parentNode;
    scalable[i].style.transform = 'scale(1)';
    var scalableContainerWidth = scalableContainer.offsetWidth - margin;
    var scalableWidth = scalable[i].offsetWidth;
    scalable[i].style.transform = 'scale(' + scalableContainerWidth / scalableWidth + ')';
    scalableContainer.style.height = scalable[i].getBoundingClientRect().height + 'px';
  }
}

Working demo: https://codepen.io/maciejkorsan/pen/BWLryj

---

This web component changes the font size so the inner text width matches the container width. Check the demo.

You can use it like this:

<full-width-text>Lorem Ipsum</full-width-text>

---

Try http://simplefocus.com/flowtype/. This is what I use for my sites, and it has worked perfectly.

---

You may be you looking for something like this:

http://jsfiddle.net/sijav/dGsC9/4/
http://fiddle.jshell.net/sijav/dGsC9/4/show/

I have used flowtype, and it's working great (however it's JavaScript and not a pure CSS solution):

$('body').flowtype({
    minFont: 10,
    maxFont: 40,
    minimum: 500,
    maximum: 1200,
    fontRatio: 70
});

---

My own solution, jQuery-based, works by gradually increasing the font size until the container gets a big increase in height (meaning it got a line break).

It's pretty simple, but works fairly well, and it is very easy to use. You don't have to know anything about the font being used, everything is taken care of by the browser.

You can play with it on http://jsfiddle.net/tubededentifrice/u5y15d0L/2/

The magic happens here:

var setMaxTextSize=function(jElement) {
    // Get and set the font size into data for reuse upon resize
    var fontSize=parseInt(jElement.data(quickFitFontSizeData)) || parseInt(jElement.css("font-size"));
    jElement.data(quickFitFontSizeData, fontSize);

    // Gradually increase font size until the element gets a big increase in height (i.e. line break)
    var i = 0;
    var previousHeight;
    do
    {
        previousHeight=jElement.height();
        jElement.css("font-size", "" + (++fontSize) + "px");
    }
    while(i++ < 300 && jElement.height()-previousHeight < fontSize/2)

    // Finally, go back before the increase in height and set the element as resized by adding quickFitSetClass
    fontSize -= 1;
    jElement.addClass(quickFitSetClass).css("font-size", "" + fontSize + "px");

    return fontSize;
};

---

I was very frustrated trying to achieve a fitty-like tight text wrapping so I ended up using a canvas-based method which I arrived at by unsuccessfully trying other methods. What I was aiming for looks like the attached which turns out to be surprisingly difficult (for me). Hopefully one day we will have a simple CSS-only way of doing this. Downsides of this approach is the text is treated more like an image, but for some use cases this is fine.

https://codesandbox.io/s/create-a-canvas-tightly-holding-a-word-st2h1?file=/index.html

This image is a screenshot of a CSS Grid layout of four full-bleed canvases.

https://i.stack.imgur.com/jhnuV.png

---

Inside your CSS, try adding this at the bottom changing the 320 pixels width for wherever your design starts breaking:

@media only screen and (max-width: 320px) {
  body { font-size: 1em; }
}

Then give the font-size in "px" or "em" as you wish.

---

My problem was similar, but related to scaling text within a heading. I tried Fit Font, but I needed to toggle the compressor to get any results, since it was solving a slightly different problem, as was Text Flow.

So I wrote my own little plugin that reduced the font size to fit the container, assuming you have overflow: hidden and white-space: nowrap so that even if reducing the font to the minimum doesn't allow showing the full heading, it just cuts off what it can show.

(function($) {

  // Reduces the size of text in the element to fit the parent.
  $.fn.reduceTextSize = function(options) {
    options = $.extend({
      minFontSize: 10
    }, options);

    function checkWidth(em) {
      var $em = $(em);
      var oldPosition = $em.css('position');
      $em.css('position', 'absolute');
      var width = $em.width();
      $em.css('position', oldPosition);
      return width;
    }

    return this.each(function(){
      var $this = $(this);
      var $parent = $this.parent();
      var prevFontSize;
      while (checkWidth($this) > $parent.width()) {
        var currentFontSize = parseInt($this.css('font-size').replace('px', ''));
        // Stop looping if min font size reached, or font size did not change last iteration.
        if (isNaN(currentFontSize) || currentFontSize <= options.minFontSize ||
            prevFontSize && prevFontSize == currentFontSize) {
          break;
        }
        prevFontSize = currentFontSize;
        $this.css('font-size', (currentFontSize - 1) + 'px');
      }
    });
  };
})(jQuery);

---

Try to use the fitText plugin, because Viewport sizes isn't the solution of this problem.

Just add the library:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>

And change font-size for correct by settings the coefficient of text:

$("#text_div").fitText(0.8);

You can set maximum and minimum values of text:

$("#text_div").fitText(0.8, { minFontSize: '12px', maxFontSize: '36px' });

---

Always have your element with this attribute:

JavaScript: element.style.fontSize = "100%";

or

CSS: style = "font-size: 100%;"

When you go fullscreen, you should already have a scale variable calculated (scale > 1 or scale = 1). Then, on fullscreen:

document.body.style.fontSize = (scale * 100) + "%";

It works nicely with little code.

---

Take look at my code. It makes the font size smaller to fit whatever there.

But I think this doesn't lead to a good user experience

var containerWidth = $("#ui-id-2").width();
var items = $(".quickSearchAutocomplete .ui-menu-item");
var fontSize = 16;

items.each(function(){
    // Displaying a value depends sometimes on your case. You may make it block or inline-table instead of inline-block or whatever value that make the div take overflow width.
    $(this).css({"whiteSpace": "nowrap", "display": "inline-block"});
    while ($(this).width() > containerWidth){
         console.log("$(this).width()" + $(this).width() + "containerWidth" + containerWidth)
         $(this).css("font-size", fontSize -= 0.5);
    }
});

https://i.stack.imgur.com/AIIPr.png

---

For dynamic text, this plugin is quite useful:

http://freqdec.github.io/slabText/

Simply add CSS:

.slabtexted .slabtext
{
    display: -moz-inline-box;
    display: inline-block;
    white-space: nowrap;
}
.slabtextinactive .slabtext
{
    display: inline;
    white-space: normal;
    font-size: 1em !important;
    letter-spacing: inherit !important;
    word-spacing: inherit !important;
    *letter-spacing: normal !important;
    *word-spacing: normal !important;
}
.slabtextdone .slabtext
{
    display: block;
}

And the script:

$('#mydiv').slabText();

---

This worked for me:

I try to approximate font-size based on a width/height got from setting `font-size: 10px`. Basically, the idea is "if I have 20 pixels width and 11 pixels height with `font-size: 10px`, so what would it be the maximum font-size to math a container of 50 pixels width and 30 pixels height?"

The answer is a double proportion system:

{ 20:10=50:X, 11:10=30:Y } = { X= (10*50)/20, Y= (10*30)/11 }

Now X is a font-size that will match width, and Y is a font-size that will match height; take the smallest value

function getMaxFontSizeApprox(el){
    var fontSize = 10;
    var p = el.parentNode;

    var parent_h = p.offsetHeight ? p.offsetHeight : p.style.pixelHeight;
    if(!parent_h)
        parent_h = 0;

    var parent_w = p.offsetHeight ? p.offsetWidth : p.style.pixelWidth;
    if(!parent_w)
        parent_w = 0;

    el.style.fontSize = fontSize + "px";

    var el_h = el.offsetHeight ? el.offsetHeight : el.style.pixelHeight;
    if(!el_h)
        el_h = 0;

    var el_w = el.offsetHeight ? el.offsetWidth : el.style.pixelWidth;
    if(!el_w)
        el_w = 0;

    // 0.5 is the error on the measure that JavaScript does
    // if the real measure had been 12.49 px => JavaScript would have said 12px
    // so we think about the worst case when could have, we add 0.5 to 
    // compensate the round error
    var fs1 = (fontSize*(parent_w + 0.5))/(el_w + 0.5);
    var fs2 = (fontSize*(parent_h) + 0.5)/(el_h + 0.5);

    fontSize = Math.floor(Math.min(fs1,fs2));
    el.style.fontSize = fontSize + "px";
    return fontSize;
}

NB: the argument of the function must be a span element or an element which is smaller than its parent, otherwise if children and parent have both the same width/height function will fail.

---

HTML

<div style="height:100px; width:200px;">
  <div id='qwe'>
    test
  </div>
</div>

JavaScript code for maximizing font-size:

var fontSize, maxHeight, maxWidth, textElement, parentElement;
textElement = document.getElementById('qwe');
parentElement = textElement.parentElement;    
maxHeight = parentElement.clientHeight;
maxWidth = parentElement.clientWidth;
fontSize = maxHeight;
var minFS = 3, maxFS = fontSize;
while (fontSize != minFS) {
  textElement.style.fontSize = `${fontSize}px`;
  if (textElement.offsetHeight < maxHeight && textElement.offsetWidth <= maxWidth) {
    minFS = fontSize;
  } else{
    maxFS = fontSize;
  }
  fontSize = Math.floor((minFS + maxFS)/2);
}
textElement.style.fontSize = `${minFS}px`;

---

As a JavaScript fallback (or your sole solution), you can use my jQuery Scalem plugin, which lets you scale relative to the parent element (container) by passing the reference option.

---

In case it's helpful to anyone, most of the solutions in this thread were wrapping text into multiple lines, form e.

But then I found this, and it worked:

https://github.com/chunksnbits/jquery-quickfit

Example usage:

$('.someText').quickfit({max:50,tolerance:.4})

---

In order to make font-size fit its container, rather than the window, see the resizeFont() function I have shared in this question (a combination of other answers, most of which are already linked here). It is triggered using window.addEventListener('resize', resizeFont);.

Vanilla JavaScript: Resize font-awesome to fit container

JavaScript:

function resizeFont() {
  var elements  = document.getElementsByClassName('resize');
  console.log(elements);
  if (elements.length < 0) {
    return;
  }
  _len = elements.length;
  for (_i = 0; _i < _len; _i++) {
    var el = elements[_i];
    el.style.fontSize = "100%";
    for (var size = 100; el.scrollHeight > el.clientHeight; size -= 10) {
      el.style.fontSize = size + '%';
    }
  }
}

You could perhaps use vw/vh as a fallback, so you dynamically assign em or rem units using JavaScript, ensuring that the fonts do scale to the window if JavaScript is disabled.

Apply the .resize class to all elements containing text you wish to be scaled.

Trigger the function prior to adding the window resize event listener. Then, any text which doesn't fit its container will be scaled down when the page loads, as well as when it is resized.

NOTE: The default font-size must be set to either em,rem or % to achieve proper results.

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