For the last decade we've supported uploading SVG vector images to MediaWiki, but we serve them as rasterized PNGs to browsers. Recently, display resolutions are going up and up, but so is concern about low-bandwidth mobile users.
This means we'd like sharper icons and diagrams on high-density phone displays, but are leery of adding extra srcset entries with 3x or 4x size PNGs which could become very large. (In fact currently MobileFrontend strips even the 1.5x and 2x renderings we have now, making diagrams very blurry on many mobile devices. See https://phabricator.wikimedia.org/T133496 - fix in works.)
Here's the base bug for SVG client side rendering: https://phabricator.wikimedia.org/T5593 I've turned it into an "epic" story tracking task and hung some blocking tasks off it; see those for more details.
TL;DR stop reading here. ;)
One of the basic problems in the past was reliably showing them natively in an <img>, with the same behavior as before, without using JavaScript hacks or breaking the hamlet caching layer. This is neatly resolved for current browsers by using the "srcset" attribute -- the same one we use to specify higher-resolution rasterizations. If instead of PNGs at 1.5x and 2x density, we specify an SVG at 1x, the SVG will be loaded instead of the default PNG.
Since all srcset-supporting browsers allow SVG in <img> this should "just work", and will be more compatible than using the experimental <picture> element or the classic <object> which deals with events differently. Older browsers will still see the PNG, and we can tweak the jquery.hidpi srcset polyfill to test for SVG support to avoid breaking on some older browsers.
This should let us start testing client-side SVG via a beta feature (with parser cache split on the user pref) at which point we can gather more real-world feedback on performance and compatibility issues.
Rendering consistency across browser engines is a concern. Supposedly modern browsers are more consistent than librsvg but we haven't done a compatibility survey to confirm this or identify problematic constructs. This is probably worth doing.
Performance is a big question. While clean simple SVGs are often nice and small and efficient, it's also easy to make a HUGEly detailed SVG that is much larger than the rasterized PNGs. Or a fairly simple small file may still render slowly due to use of filters.
So we probably want to provide good tools for our editors and image authors to help optimize their files. Show the renderings and the bandwidth balance versus rasterization; maybe provide in-wiki implementation of svgo or other lossy optimizer tools. Warn about things that are large or render slowly. Maybe provide a switch to run particular files through rasterization always.
And we'll almost certainly want to strip comments and white space to save bandwidth on page views, while retaining them all in the source file for download and reediting.
Feature parity also needs more work. Localized text in SVGs is supported with our server side rendering but this won't be reliable in the client; which means we'll want to perform a server side transformation that creates per-language "thumbnail" SVGs. Fonts for internationalized text are a big deal, and may require similar transformations if we want to serve them... Which may mean additional complications and bandwidth usage.
And then there are long term goals of taking more advantage of SVGs dynamic nature -- making things animated or interactive. That's a much bigger question and has implementation and security issues!
-- brion