Save bandwidth with webP – soon with fallback! Tuesday 12 March 2013

A long time ago, “responsive” didn’t mean “resize your browser window repeatedly while fellow designers orgasm until they resemble a moleskine atop a puddle”. It simply meant “Reacting quickly and positively”, meaning that the page loaded fast and you could interact with it immediately.

One way to do this is to reduce the weight of the page by serving images that have a smaller file-size, thereby consuming less bandwidth and taking less time to download a page. In the last year, web pages download approximately the same number of images, but their total size has increased from about 600K to 812K, making images about 60% of the total page size.

One way to reduce this amount is to encode images in a new(ish) format called webP. It’s developed by Google and is basically a still version of their webM video codec. Google says

WebP is a new image format that provides lossless and lossy compression for images on the web. WebP lossless images are 26% smaller in size compared to PNGs. WebP lossy images are 25-34% smaller in size compared to JPEG images at equivalent SSIM index. WebP supports lossless transparency (also known as alpha channel) with just 22% additional bytes. Transparency is also supported with lossy compression and typically provides 3x smaller file sizes compared to PNG when lossy compression is acceptable for the red/green/blue color channels.

Opera uses it precisely for this compression; it’s used in Opera Turbo, which can be enabled in Opera desktop, Opera Mobile and the Chromium-based Yandex browser. This transcodes images on-the-fly to webP before squirting them down the wire and, on slower connections, it’s still faster.

In tests, Yoav Weiss reported that “Using WebP would increase the savings to 61% of image data”.

WebP is currently supported only in Opera (Presto), Google Chrome, Yandex and Android Browser on Ice Cream Sandwich, which makes it difficult to deploy on the Web. Firefox doesn’t like it and IE hasn’t said anything (I wonder if the new confidence about technologies in the VP8 video codec on which it’s based might make them feel better about it?)

However, there’s some handy new CSS coming to the rescue soon (when browser vendors implement it). We’ve long been able to specify CSS background images using background-image: url(foo.png); , but now say hello to CSS Image Values and Replaced Content Module Level 4’s Image Fallbacks, which uses this syntax:

background-image: image("wavy.webp", "wavy.png", "wavy.gif");

(Note image rather than url before the list of images.)

The spec says “Multiple ‘image -srcs’ can be given separated by commas, in which case the function represents the first image that’s not an invalid image.”

Simply: go through the list of images and grab the first you can use. If it 404s, continue going through the list until you find one you can use. Note that this isn’t supported anywhere yet, but I hope to see it soon.

[Added after a reminder from Yoav Weiss:] It needs finessing too; Jake Archibald points out “If the browser doesn’t support webp it will still download ‘whatever.webp’ and attempt a decode before it’ll fallback to the png” and suggests adding a format() qualifier, from @font-face:

background-image: image("whatever.webp" format('webp'), "whatever.jpg");

But what about old [current] browsers?, I hear you ask. Give them the current url syntax as fallback:

background-image: url("wavy.gif");

background-image: image("wavy.webp", "wavy.png", "wavy.gif");



Now all browsers get a background image, and those that are clever enough to understand webP get smaller images. Of course, you have to make a webP version (there are webP conversion tools, including a Photoshop plugin).

It seems to me that the spec is overly restrictive, as it seems to require the browser to use the first image that it can. webP is heavily compressed so requires more CPU to decode than traditional image formats. Therefore, I could imagine a browser that knows it’s on WiFi and using battery (not plugged in) to choose not to use webP and choose a PNG/ JPG etc to save CPU cycles, even though the file-size is likely to be larger.

What about content images?

Of course, not all images on your webpages are CSS background images. Many are content images in <img> elements, which doesn’t allow fallbacks.

There is, however, an HTML5 element that deliberately allows different source files to get over the fact that browsers understand different media formats:

<video>

<source src=foo.webm type=video/webm>

<source src=foo.mp4 type=video/mp4>

... fallback content ...

</video>

Wouldn’t it be great if we could use this model for a New! Improved! <img> element? We couldn’t call it <image> as that would be too confusing and the HTML5 parser algorithm aliases <image> to <img> (thanks Alcohi). So for the sake of thought experimentation, let’s call it <picture> (or, if we’re bikeshedding, <pic> or —my favourite— <bruce>). Then we could have

<picture>

<source src=foo.webp type=image/webp>

<source src=foo.png type=image/png>

<img src=foo.png alt="insert alt text here"> <!-- fallback content -->

</picture>

And everyone gets their images, and some get them much faster.

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