I use Opera 9.63. This browser can send received HTML code to http://validator.w3.org. That validator shows 63 Errors, 214 warnings. I have very long horizontal scroll bar, about 3.5 times of html page width.
Most of those errors (not to say all) come from the syndicated posts, so I guess it doesn't really matter in a planet service. I may try to find errors of our html files, but it is not so trivial. Thanks for the report though. Please reopen if you want to point problems in our files only. http://sources.gentoo.org/viewcvs.py/planet/templates/