Facebook Connect and their "Social Widgets" documentation mention that you need to add an xmlns attribute to your <html> tag on the page where it will be used.
I understand that xmlns is for XML Name-spacing, and have used such with XHTML before. However, with all the recent talk about HTML4 / HTML5, without having read through the entire spec, is the xmlns attribute compatible with valid HTML5? What about HTML4?
If I've looked over an obvious mention of this in the docs, I'm sorry... point it out?
EDIT: A couple docs references/cites:
The below answer is from 2009. It discusses the state of HTML 5 at the time.
The above question is also from 2009. It discusses the state of the Facebook APIs at the time.
Neither are relevent for 2017.
is the xmlns property
It's an attribute.
compatible with valid HTML5?
Sort of. It is completely meaningless, but allowed so people can be lazy when porting XHTML.
Remember that HTML 5 is an unfinished draft. These things are subject to change. Don't rush into using HTML 5 unless it offers a serious benefit.
What about HTML4?
Not at all.