I'm inferring that by talking about becoming an expert that you already know some HTML and CSS and are therefore intermediate or knowledgeable more than that. In case you do not know any HTML and CSS at all, try W3SCHOOLS
HTML is not as big as CSS since it is CSS that you have to tweak most of the time for layout and cross-browser compatibilty.
So for CSS, try this Monsterlist --> http://bit.ly/23Yd5J
. If you can master about 5% of the stuff in that collection, I think that you can call yourself an expert and declare that you have quit learning.