background-size polyfill

An IE behavior adding support for background-size: cover; and background-size: contain; to IE8.

Demo

Introduction

Progressive Enhancement is the mantra I live by. It means "Have fun with CSS3 and don't worry about IE8 users; they'll never notice they're missing out on your gorgeous text-shadows and gradients, anyway".

All was well until I discovered the elegance of background-size: cover; and background-size: contain;. The first one, for instance, allows an image to completely cover a background, without having to send a 1920x1080 background image down the pipes.

Unfortunately, they don't degrade gracefully: websites would likely appear broken to IE8 users :-( ...unless you use this behavior!

How to use it?

Upload backgroundsize.min.htc to your website, along with the .htaccess that will send the mime-type required by IE (Apache only — it's built in nginx, node and IIS).

Everywhere you use background-size: cover; or background-size: contain; in your CSS, add a reference to this file.

.selector { 
	background-size: cover;
	/* The url is relative to the document, not to the css file! */
	/* Prefer absolute urls to avoid confusion. */
	-ms-behavior: url(/backgroundsize.min.htc);
}

The elements styled this way should have a position: relative; or position: fixed; and a z-index. If not, they will be given a position: relative; and z-index: 0;.

Limitations

This polyfill inserts an <img/> in the background of all elements matched in the css to emulate the cover or contain value. Calculating the displayed position and size of this image is quite complex and function of numerous parameters:

• the size of the element itself
• the size of the image
• the values of background-[size/position/clip/origin/attachment/scroll]

It is thus impossible to emulate background-size completely and perfectly. But it's still possible to enjoy the main features:

• correct position and size of the background image
• updated position and size on browser resize
• updated image, position and size when the background-image is modified

The following style properties, values or behavior aren't supported:

• values other than cover or contain in background-size
• multiple backgrounds (although the :after trick can still be used)
• 4 values syntax of background-position
• lengths (px, em, etc.) in background-position (only percentages and keywords such as center work)
• any repeat value in background-repeat
• non-default values of background-[clip/origin/attachment/scroll]
• resizing the background when the dimensions of the element change

Removing any of these limitations is probably just one fork away...

License

MIT Licensed http://louisremi.mit-license.org/, by @louis_remi

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