HTML has a lot of structure already (for example all levels of heading are easy to pick out, lists are easy to pick out), and Google does encourage use of semantic tags (for example for review scores, or author details, or hotel details). For most searches I don't think the problem lies with being able to read meaning - the problem is you can't trust the page author to tell you what the page is about, or link to the right pages, because spammers lie. Semantic tags don't help with that at all and it's a hard problem to differentiate spam and good content for a given reader - the reader might not even know the difference.