I swear I've seen the mime for a page declared some place in Opera but I just can not seem to find where it was. Also how far back can we determine the mime of a page in Opera? I know Opera 6 supports application/xhtml+xml and I recall Opera 5 also having support.
For application/xhtml+xml support, Opera only handles it correctly since version 7.10 (Windows), 6.1 (Linux/Unix) and 6.02 (Mac). For most version 6.x versions the mime type is recognized but handled incorrectly (case-insensitive). 5.x had no support. See this chart:
There was also something stashed away in one of the panels...that F4 key panel maybe? Also the drop down bookmarks icon isn't accessible once you delete it from one of the panels underneath the address bar.
The Opera GUI in short while offering a lot really needs to get much more organized.