Is it bad to use uppercase letters for html tags?

Question

What is the best practice?

  or 

And why we should stick with one particular case?

However all browsers see

Answer 1

The only relevant parts of specifications say:

• HTML tag and attribute names are case insensitive.
• XHTML tag and attribute names are case sensitive and must be lower case

Stuff that isn't mentioned in any standard:

• Lower case is generally considered easier to read
• Lower case is most common (and what people are used to working with)
• Holding down the shift key or toggling CAPS LOCK all the time is a pain