What do the crossed style properties in Google Chrome devtools mean?

When a CSS property shows as struck-through, it means that the crossed-out style was applied, but then overridden by a more specific selector, a more local rule, or by a later property within the same rule.

(Special cases: a style will also be shown as struck-through if a style exists in an matching rule but is commented out, or if you've manually disabled it by unchecking it within the Chrome developer tools. It will also show as crossed out, but with an error icon, if the style has a syntax error.)

For example, if a background color was applied to all divs, but a different background color was applied to divs with a certain id, the first color will show up but will be crossed out, as the second color has replaced it (in the property list for the div with that id).

In addition to the above answer I also want to highlight a case of striked out property which really surprised me.

If you are adding a background image to a div :

<div class = "myBackground">

</div>

You want to scale the image to fit in the dimensions of the div so this would be your normal class definition.

.myBackground {

 height:100px;
 width:100px;
 background: url("/img/bck/myImage.jpg") no-repeat; 
 background-size: contain;

}

but if you interchange the order as :-

.myBackground {
 height:100px;
 width:100px;
 background-size: contain;  //before the background
 background: url("/img/bck/myImage.jpg") no-repeat; 
}

then in chrome you ll see background-size as striked out. I am not sure why this is , but yeah you dont want to mess with it.

On a side note. If you are using @media queries (such as @media screen (max-width:500px)) pay particular attention to applying @media query AFTER you are done with normal styles. Because @media query will be crossed out (even though it is more specific) if followed by css that manipulates the same elements. Example:

@media (max-width:750px){
#buy-box {width: 300px;}
}

#buy-box{
width:500px;
}

** width will be 500px and 300px will be crossed out in Developer Tools. **

#buy-box{
width:500px;
}

@media (max-width:750px){
#buy-box {width: 300px;}
}

** width will be 300px and 500px will be crossed out **

If you want to apply the style even after getting struck-trough indication, you can use "!important" to enforce the style. It may not be a right solution but solve the problem.

There are two ways to know which rules are overriding:

Search the property in the Filter box at the top of the Styles tab. It will show all the rules containing that property, with the property highlighted in yellow. Look in the Computed tab to find the same property type, and then expand that to see the source of the various rules that are trying to apply that property.

This happens also if you forget to set the Unit of the value.
For example:
margin-left: -53
instead of
margin-left: -53px;

There is some cases when you copy and paste the CSS code in somewhere and it breaks the format so Chrome show the yellow warning. You should try to reformat the CSS code again and it should be fine.