Not true. The following sentence is enclosed in <BigSkyFreeper> tags.
Test.
Did you get an error or is sentence displayed and the tag just ignored?
You can't claim that IE "supports" the <BigSkyFreeper> tag can you? IE handles it and the <blink> tag the same way after all.
Just shows up as "less than" and "more than" signs in FF and IE, because it's not a valid tag in any browser, and hasn't been defined as a tag by the "document creator" in this case FR. So, even your example is flawed.