Notepad is a text editor; naturally it will not render html, but will display it as text. That is the whole point of a text editor. Neither will it offer you the option of saving text as html (unless you have opened an existing html file in Notepad and then selected the 'save as' option; in which case the original filename, with extension, will appear by default), any more than it will save plain text in a binary format such as rtf.

Since html is just a text file with an .html extension (or .htm if you prefer microsoft proprietory usage), you have only to follow the simple instructions given much earlier in this surprisingly long thread to achieve the desired result.

It really is childishly simple, you know.