It's not that hard to become good with HTML cause there ain't that many tags... I guess it depends on what you mean though? I've been writing HTML for 10 years and I probably wouldn't consider myself an expert.
You can call yourself an expert after you memorize all this: W3C - HTML 4.01 Specification W3C - CSS 2.1 Specification But you still won't get any respect without heaps of sites in your portfolio. The browsers don't always follow the specs, and you're not a true expect until you have mastered that nightmare
Seriously. Throw in browser incompatibilities and I think that it becomes impossible. You can get good, but it's fluid.
What is a expert anyway? Some wise man once said :"Someone calling themselves an expert is someone who has made all the mistakes there can be made in a particular industry." Wise thought. Be wary of experts and listen to people that quietly go abut succeeding everyday at what they do and don't write books and manuals to brag about what kind of experts they are.
fact is to become expert in any area you need study, study, study. For xhtml/css-coder you need to learn W3C specifiations, to know difference between browsers, some aspects of AJAX, JS, PHP, XML, MYSQL at least, to have some knowledge of mobile browsers. And all that changes EVERYDAY!
The fact is that you can learn HTML fairly quickly, it's not that complicated. But when you start trying to actually put together websites, then you run into browser inconsistencies, quirks, bugs, etc... Really becoming an expert means understanding the medium and learning to deal with all of it's inherent problems. And that could take a lifetime.
You don't need to know much HTML these days. Programs like dreamweaver code for you, just not very well.
You should still know HTML if you want to be a programmer. Make PHP & CSS your emphasis, then get a good grip on HTML and you'll be fine.
and code, code code. Riftshighway seconded: still learning after a bit over a year. I think I have an excellent grasp, and would call my (current, not old) code professional, but I still run into new things all the time.