Here is a list of the most common mistakes made in HTML code. You can find more mistakes and details here.
Spelling
Always check you have spelled everything precisely
HTML uses American spelling, like "center" and "color", not "centre" and "colour"
Speech Marks
Always put speech marks round the values of a parameter.
For example in bgcolor="red", "red" is the value and needs speech marks round it.
HTML only likes straight speech marks (") not curly ones(“).
To get sraight speech marks, always type them into TextEdit or Notepad.
Don't copy them from elsewhere.
You can use double speech marks(") or single ones(') - HTML doesn't mind
Tags
Always make sure every pair tag (which needs a partner) has a partner (eg <h1>..</h1>).
Make sure the slashes go the right way. In tags and file and web addresses, slashes are "forward slashes" and go /.
Tags can be written all on one line or on separate lines, or with spaces - HTML doesn't mind.