In the current standard, the tags are lower case. But browsers usually let you use upper case.
The example Web pages such as the one below
never become files on your hard disk.
On your computer they exist
only in the main memory being used by the browser.
As a security feature,
browsers do not read or write files to your hard disk
unless you ask it to.
Use File/Save As
in the menu bar.
Or you can copy text from any Web page into Notepad and
save the text to a disk file:
Once you have done this, you can look at the file with your browser
(navigate to it using File/open
or File/open page
from the menu)
or click on it in Windows explorer.