Accidentally putting the ">" or "/" the wrong way or taking hours to figure out what was causing the syntax error when it was simply a (somehow) backwards quotations mark fault. Little mistakes makes the big time wasters.

Damn. I quit school because of that.

If its just HTML (not CSS/PHP), then I'd say organization. Now, including other languages, then using MySQL tables with Php is impossible with me.

To me the hardest part is using and organizing everyone else organization then fixing broken coding.

If you use the syntax correctly, it doesn't get jumbled up at all.
I've been doing HTML/CSS/JavaScript and all that jazz for more than 8 years, and the IMO hardest part to learn was all the new HTML 5 goodies when it came out. It's a very simple languages, and also very forgiving, If you need any help with any, shoot me a msg, I'll be glad to help.

If you organise your code neatly and indent it correctly it can be very easy to read.
HTML itself is not a difficult language at all, using CSS, Javascript, MySQL and other languages along side it is when it gets confusing and more difficult.

To me, the most difficult part is thinking of what I want to create.