The new Clearspace 2.5 Rich Text Editor (RTE) is a huge leap in functionality over previous versions. You can edit the pure html by clicking the "HTML" link in the top right corner of the RTE. As before though, there is a default html filter that will remove html that has a risk of allowing cross site scripting attacks. You can adjust the html filter by going in to your admin console > Spaces > Settings > Filters and Macros and then look for HTML Filter under Post Processing Filters. There are a lot of settings to control what is allowed.
Header 1 Tables with Headers are well supported and there are a lot of options
There are also some very cool tie-ins to the plugin api that allow you to create macros that will appear in the toolbar at the top of the RTE. You can read more about the new RTE by searching the Jivespace community, but you can also find some information on Adam Wolf's site in a post about new 2.5 features.
Jive Certified Clearspace Developer
Thank you so much, Joe, I appreciate your taking the time to put me in the Rich Text picture. I look forward to experimenting with the new editor once we complete our upgrade this month!