On my site, EVERYTHING I put in the Write Post has HTML and XHTML. EVERYTHING.
I use the TextControl plugin to manually set the generated formatting results on a per post basis. Since I almost always include massive HTML in my posts, I have also set the global settings to not format the posts and to rely upon the HTML for presentation.
Tables in design are BAD (she screams to the world but no one listens ?? ) and not for design. Tables are for DATA only. So if you are including a table for data, like the population numbers for four European cities, then you can either give the table a “class” and add that to your style sheet as a permanent “look” for all of those tables, or (ugh) just manually add the border and colors and so on.
What might be happening is the wpautop format that is the default for posts within WordPress, whose job is to turn non HTML writing into HTML, is converting your tags. It has nothing to do with everything we’ve already discussed, to a point. You want to control the use of HTML, use TextControl.