sux at webdesign

Halp, how do I learn to write HTML web-pages that are worth a shit? ie. look pretty well the same across browsers, will not do stupid things if you resize the window, etc.  I know how to use CSS to separate content and presentation and all that jazz

Forget CSS, just use semantic HTML5.
Visual appearance has no relevance whatsoever to your data and is purely a client issue: if some clients want the web to look ``pretty'', they'll create user stylesheets. Most people don't--their fault.  
The onus is on the users who want ``pretty'' web sites to use a suitable client, not on HTML authors to provide extraneous styling files. 
In fact, you might want to look at your server access log: if you write for an audience of intelligent people, you will see that their favorite user-agents will be Mozilla Firefox (which supports user stylesheets through the ``stylish'' extension), Opera (which supports them natively); links, w3-mode, lynx, and w3m (these four browsers do not benefit from CSS--and even if they did, some of those are often used in conjunction with a mail daemon that sends the contents of the page as plain text, for users who prefer not to browse the web from their main computer).