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

I prefer lowercase, because that is what is used by the HTML 5.2 specification: https://www.w3.org/TR/html52/introduction.html#a-quick-introduction-to-html

The HTML 4.0 specification shows HTML tags in uppercase, but the specification itself uses lowercase tags in its HTML code: https://www.w3.org/TR/html40/struct/global.html

Java's javax.swing.text.html.Html.Tag class uses lowercase (even though the Java constants are in uppercase, as is consistent with Java's naming conventions).

Go's godoc tool uses lowercase.