An html file is just a text file where some sequences of characters have a predefined meaning.
There's no such thing as a standardised html log file format, so only you know what the format of that html log file should be. To produce a valid html (5) file, it could be as simple as writing:
<!DOCTYPE html>
<html><body><p>
before the text you normally write to the log file and
</p></body></html>
after.
Note that html is not a practical format for sharing data between computers, it's only useful for displaying things in a web browser. For sharing data between computers, you have XML (or json, or ...).
