An html document is, in fact, written in plain text. It is just a code that tells a Web browser how to represent a page. You can take a look at the code using the View Page Source function of your browser. For most pages these days it will look horribly complex. Here is an example of a simple Web page as seen in a text editor, where I have replaced the <> in the code with a [] so it will actually show as text here:
[!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"]
HTML Code:
[HEAD]
[TITLE]Title of the web page [/TITLE]
[/HEAD]
[BODY]
An example of a simple [B]Web[/B] page.
[/BODY]
If I had not changed the code, replacing <> by []. here is how it would look in this message (because you are looking at it in a Web browser):
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<HTML>
<HEAD>
<TITLE>Title of the web page </TITLE>
</HEAD>
<BODY>
An example of a simple
Web page.
</BODY>
</HTML>