Yeah, but then somebody would come and say, "when you write <i>foo</i>, what you really wanted to do is to emphasize", and so you'll write <em>foo</em> instead, and leave the italicizing out as an implementation detail.

Implementation details still have to be implemented. Someone else might come along and figure that different documents may want to interpret "emphasis" differently, and while we're at it, why not make it composable? They may design a DSL for that, and give it a cool name too, like "cascading style sheets".