Does anybody know how to convert html files to avi? Or another file tye?

html videos are already downloadable, if not then how are you viewing them in your browser?

I suppose your query is, how to download one that is embedded in the webpage.

In that case use,

flashgot: http://flashgot.net/

if you are using Firefox: http://www.mozilla.org/en-US/firefox/new/

or, free download manager: http://www.freedownloadmanager.org/

it has integration support with all the major browsers.

or, search: https://duckduckgo.com/?q=download+manager

most of them has browser integration support and ways to download embedded videos.

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Html files are text files that are understood by webbrowsers. You cannot make a video file out of a text file. You can rename it to ".txt" to see the content.