This is closer to the mark: implementations of CSS cause a lot of problems. CSS isn&#x27;t obviously worse than mobile frameworks though. In fact, I&#x27;m getting fairly confident that CSS can potentially achieve <i>better</i> performance than UIKit and the Android stack (because it&#x27;s parallelizable and declarative as opposed to procedural, neither of which apply to mobile app frameworks), though of course browsers of today don&#x27;t take advantage of this well.<p>Another large issue is that browser implementations of CSS are a black box: very few Web developers understand how it works and browsers present a hidden maze of slow paths. (Which CSS animations will run in the compositor? Which run a separate thread? What events cause off main thread scrolling to be disabled? What order will the browser fire events in?) This is, again, an issue that&#x27;s more about implementation more than anything: I believe a better from-scratch implementation of the CSS specs would fix a lot of this.