[billions]

Webkit was the only thing the mobile web had that apps did not: a mostly un-fragmented codebase. Now developers will have to test their mobile web-apps against even more browsers reducing time-to-market efficiency. Hopefully competition will lead to faster improvements by top players (Google), but the more likely scenario is the least incentivized player (Apple) will drag their feet re-creating an IE6 scenario where one browser will slow mass adaption of standards.

[joe_fishfish]

Mobile web developers should already be used to testing their work on multiple different devices anyway. The deployed versions of WebKit differ quite significantly across the myriad of mobile devices on the market, and WebKit is just the layout engine anyway. It doesn't run JS or talk directly to the network stacks, and it isn't responsible for any GPU acceleration that may or may not be available.

If you're doing anything more complicated than serving static HTML and images to mobile devices you will already need to test on as many different devices as is feasible for you. It's basically unheard of for something to work across all WebKit based browsers just because your CI is telling you it works OK in desktop Chrome.