PSA - "INVALID HTML" MESSAGE
If you are receiving this message please don't freak out there are a number of reasons (such as external scripts, mp3 scripts, tumblr itself, and lastly the theme code) for this so take the following steps to make sure it is NOT your theme.
Wait. Sometimes for a brief moment it will flash Invalid HTML but then it will return to normal.
Save anyways, refresh the page. Sometimes tumblr just glitches out and needs to "catch up". THIS IS WHY YOU SHOULD ALWAYS USE A TEST BLOG FIRST.
Try seeing if it is something in your description or MP3 player. Tumblr can get testy when it comes to external scripts, so remove those paste it somewhere to save, save the theme without a description and see if it removes it.
Sometimes if the DOCTYPE at the top is something else other than <!DOCTYPE html> tumblr can freak out. So check your theme to see if it is and maybe talk to the theme maker to see if they then still have the same problem as you.
Lastly sometimes it involves cookies/cache, so if you know how clear those out log out of tumblr, log back in and refresh the customize theme page to see if it works.
95% of the time it is just tumblr. Many times if you just simply refresh or save and restart your browser it returns to normal. Again with installing themes, the new customize page can be rather misleading so always always use a test blog just so if something does mess up, it won't effect your actual blog.