Is it all right to use <marquee> HTML element yet? I mean, is it supported by all browsers yet? I know Twitter is using it, I think it's quite good for announcements and I'd like to use it, but I don't know if it will work properly yet. Does anyone know?
No, it was DEPRECATED by the W3C.
Good luck!
EDIT:
An alternative for marquee would be to display the whole string in a span or div, for accessibility.
EDIT2:
I can't be sure but at the first glance, twitter isn't using it, BUT even if they used it (which they shouldn't) you should not follow their bad example, they can make mistakes too! :)
EDIT3:
You say it works in all browsers, while this may be true, because modern browsers try to be as compatible as possible, but you should consider that that it is good discipline to always go along with the standards.
Why? A lot of reasons: for example, the mobile browser (every day more popular) don't have the memory to spare as desktop browsers like FF Chrome and IE and they might not display toy content correctly.
EDIT4:
I just got that you were talking about "moving" trending topics, I´m really sorry for the dela, for completeness sake, here is one example:
Demo:
http://remysharp.com/demo/marquee.html
Source: