Is embedding background image data into CSS as Bas

Question

I was looking at the source of a greasemonkey userscript and noticed the following in their css:

.even { background: #fff url(data:image/gif;base64,R0lGODlhBgASALMAAOfn5+rq6uvr6+zs7O7u7vHx8fPz8/b29vj4+P39/f///wAAAAAAAAAAAAAAAAAAACwAAAAABgASAAAIMAAVCBxIsKDBgwgTDkzAsKGAhxARSJx4oKJFAxgzFtjIkYDHjwNCigxAsiSAkygDAgA7) repeat-x bottom}

I can appreciate that a greasemonkey script would want to bundle anything it can within the source as opposed to host it on a server, that's obvious enough. But since I had not seen this technique previously, I considered its use and it seems appealing for a number of reasons:

1. It will reduce the amount of HTTP requests on page load, thus enhancing performance
2. If no CDN, then it will reduce the amount of traffic generated through cookies being sent alongside of images
3. CSS files can be cached
4. CSS files can be GZIPPED

Considering that IE6 (for instance) has problems with cache for background images, this seems like it's not the worst idea...

So, is this a good or bad practice, why WOULDN'T you use it and what tools would you use to base64 encode the images?

update - results of testing

• testing with image: http://fragged.org/dev/map-shot.jpg - 133.6Kb

• test URL: http://fragged.org/dev/base64.html

• dedicated CSS file: http://fragged.org/dev/base64.css - 178.1Kb

• GZIP encoding server side

• resulting size sent to client (YSLOW components test): 59.3Kb

• Saving of data sent to client browser of: 74.3Kb

Nice, but it will be slightly less useful for smaller images, I guess.

UPDATE: Bryan McQuade, a software engineer at Google, working on PageSpeed, expressed at ChromeDevSummit 2013 that data:uris in CSS is considered a render-blocking anti-pattern for delivering critical/minimal CSS during his talk #perfmatters: Instant mobile web apps. See http://developer.chrome.com/devsummit/sessions and keep that in mind - actual slide

Answer 1

One of the things I would suggest is to have two separate stylesheets: One with your regular style definitions and another one that contains your images in base64 encoding.

You have to include the base stylesheet before the image stylesheet of course.

This way you will assure that you're regular stylesheet is downloaded and applied as soon as possible to the document, yet at the same time you profit from reduced http-requests and other benefits data-uris give you.

Answer 2

It's not a good idea when you want your images and style information to be cached separately. Also if you encode a large image or a significant number of images in to your css file it will take the browser longer to download the file leaving your site without any of the style information until the download completes. For small images that you don't intend on changing often if ever it is a fine solution.

as far as generating the base64 encoding:

• http://b64.io/
• http://www.motobit.com/util/base64-decoder-encoder.asp (upload)
• http://www.greywyvern.com/code/php/binary2base64 (from link with little tutorials underneath)

Answer 3

This answer is out of date and shouldn't be used.

1) Average latency is much faster on mobile in 2017. https://opensignal.com/reports/2016/02/usa/state-of-the-mobile-network

2) HTTP2 multiplexes https://http2.github.io/faq/#why-is-http2-multiplexed

"Data URIs" should definitely be considered for mobile sites. HTTP access over cellular networks comes with higher latency per request/response. So there are some use cases where jamming your images as data into CSS or HTML templates could be beneficial on mobile web apps. You should measure usage on a case-by-case basis -- I'm not advocating that data URIs should be used everywhere in a mobile web app.

Note that mobile browsers have limitations on total size of files that can be cached. Limits for iOS 3.2 were pretty low (25K per file), but are getting larger (100K) for newer versions of Mobile Safari. So be sure to keep an eye on your total file size when including data URIs.

http://www.yuiblog.com/blog/2010/06/28/mobile-browser-cache-limits/

Answer 4

I disagree with the recommendation to create separate CSS files for non-editorial images.

Assuming the images are for UI purposes, it's presentation layer styling, and as mentioned above, if you're doing mobile UI's its definitely a good idea to keep all styling in a single file so it can be cached once.

Answer 5

You can encode it in PHP :)

<img src="data:image/gif;base64,<?php echo base64_encode(file_get_contents("feed-icon.gif")); ?>">

Or display in our dynamic CSS.php file:

background: url("data:image/gif;base64,<?php echo base64_encode(file_get_contents("feed-icon.gif")); ?>");

1 That’s sort of a “quick-n-dirty” technique but it works. Here is another encoding method using fopen() instead of file_get_contents():

<?php // convert image to dataURL
$img_source = "feed-icon.gif"; // image path/name
$img_binary = fread(fopen($img_source, "r"), filesize($img_source));
$img_string = base64_encode($img_binary);
?>

Source

Answer 6

In my case it allows me to apply a CSS stylesheet without worrying about copying associated images, since they're already embedded inside.