Steps to reproduce:

1. Create a new article;
2. Copy a table from a website that has <thead> set to represent table headers (e.g. http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_tbody);
3. Paste the copied table into an article's rich text field, such as "body";
4. The editor correctly shows that the header is part of the table;
5. Publish the article;
6. Both <thead> and <tfoot> elements are removed from the saved content.

Please note that <tbody> tags are correctly preserved.