While playing around with some sketchs for Wemos mini D1 I found the reference to the following u...

[G+_Rud Dog]

While playing around with some sketchs for Wemos mini D1 I found the reference to the following url in its content and was wondering if anyone can tell me why it is called and its purpose.

 

https://www.w3.org/1999/xhtml/

[G+_John Sullivan]

What is the line or lines of code that calls it?

[G+_Paul Hutchinson]

It's usually part of a xmlns declaration in a XHTML document. Ref. w3.org - XHTML 1.0 - Normative Definition of XHTML 1.0