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.
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 page from the menu)
or click on it in Windows explorer.