In HTML, there is obsolete, and there is deprecated, and they are not the same thing (well, they probably are, but someone wanted to change the terminology... deprecated was supposed to mean "thrown out of the spec" while obsolete seems to mean "it's in the spec but it should not be used EVAR").
Marquee in HTML5 is, like, obsolete-obsolete*. It will throw an error in the validator. As opposed to the obsolete-but-conforming elements, who'll only give you a warning.
Well and watch out, there are two specs we're dealing with... W3C and WHATWG. An example of weirdness is the <u> element. **
Still, there is an "implementation" page for vendors (browser makers) with specific instructions on how to support these things. There's one for marquee.
Most of them do. The big non-supporter I know of is Chrome, who does support marquee, but doesn't seem to hold to the implementor's rules. Or something. It misses some attributes and features IE originally had in marquee that most of the earlier browsers has supported... I forget which ones, and I only know this because someone came over here to SitePoint and asked why his marquee wasn't doing x and y in Chrome. It was lawlz.
But, I mean, it scrolls.
WHATWG did just that. They gave it the smelly name of "paving the cowpaths". It was via this manner that <iframe>,<embed>, and XMLHttpRequest were brought into the HTML spec (iframe back from deprecation; embed who like marquee was never in the spec at all and brought in; same for XHR). They were brought in because they were so much in use, the spec writers decided there ought to be a specification for something used so much. Because specifications tell you how browser vendors (implementors) should deal with that markup and how authors should write it.
Marquee never got that because, like blink, it was probably considered a monstrosity that should have been killed while incubating over at MS.
Long ago I noticed in Firefox (back in ... 3.0?) that the page XhtmlCoder linked to, the Evan Goer's OH POINTY BIRDS page, that not all the blinking and marqueeing elements were actually moving. The browser had blocked many by default it seemed... only a few of the elements appeared. Take a look in an older browser and you'll see about twice as many.
But anyway, so what if it's deprecated? Or obsolete? That's for validators anyway. If you have such a child-eating hoster, and you are not an HTML writer, validation is not one of your worries.
** It's deprecated in the W3C HTML5 spec: http://www.w3.org/TR/html401/appendix/changes.html#idx-deprecated
It is not deprecated in the WHATWG HTML5 spec: http://www.whatwg.org/specs/web-apps/current-work/multipage/text-level-semantics.html#the-u-element