People use it to create websites
Some tools people could use to validate HTML code are Dr. Watson, Validate HTM Firefox add on and WDG HTML validator. Also popular to validate HTML codes are Cahse HTML validator life and HTML Toolbox.
You can't use PHP in an HTML document, but you can use HTML in PHP script.
I use HTML, and the majority of the people I work with and for receive HTML emails.Most of the large email providers (Yahoo!, GMail, Hotmail, etc.) display HTML email by default. Most of the same also send HTML email by default. Outlook and Thunderbird also display HTML email by default.Pretty much, HTML email is a de facto standard, despite its drawbacks.
http://www.w3schools.com/HTML/
The html converters that are free to use are HTML Converter and Free HTML Converter. These two websites do a great job at converting html files to whatever you need.
There are many sites which teach you HTML,Google ithttp://www.w3schools.com/HTML/
You can't use HTML on Facebook.
Use an html snippet
You can use some HTML for pictures and formatting, but not all HTML can be used (for security purposes). Note that you might need to play around a bit to get HTML to work.
XHTML is used as a stricter view of HTML. People who like to make the code clean and nice use XHTML.
Use the character entity for the <: &lt;
Yes. if you look at page source you will see <html> and other HTML tags.