Why doesn't html coding work on my mac?

Do you know what the only difference between a plain text file and an html file is? The file extension. When the computer sees a file that ends in .txt, it will read it literally as text. When it sees a file that ends in .html, it will read it as HTML. Try renaming the file so that it ends in .html instead of .txt. TextEdit will warn you about it, but choose to save it as a .html file anyway.