It's not really a subset, more like a dialect. It has some features that web CSS doesn't, for instance. Some things like animations use the same ideas and APIs

It's not really a subset, more like a dialect. It has some features that web CSS doesn't, for instance. Some things like animations use the same ideas and APIs but there just isn't a CSS DSL for it, you code up timelines and keyframes in the host language (usually but not always Java). The problems with CSS are more around the high levels of inconsistency, difficulty making it fast, lack of modularity and the fact that if you make a mistake either nothing happens with no error, or you get visual corruption. It's just an extremely frustrating technology to work with, especially if you only use it occasionally. Although the article talks about font metrics a lot, a big part of why centering things is so hard is thanks to CSS and its legacy which is why it opens with that. Every time I do web design I discover that the current best practice has changed, and there are dozens of new properties with similar names to existing properties which don't seem to work or can only be used in combination with other properties.