Does HTML 5 need `<html xmlns="http://www.w3.org/1999/xhtml">`

HTML5 does not require the use of the xmlns attribute as that is specific to XHTML (which means not even HTML 4 uses it either).

If you're just serving regular HTML5, then you can leave out that attribute entirely:

<!DOCTYPE html>
<html>

The xmlns attribute is only required if you're writing and serving XML-serialized HTML5, aka XHTML5.

No, you don't need to include it. This should be enough.

 <!doctype html>
 <html>

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