Comment by aerugo_

No, it's not "running" the file, it's the browser that runs it. For some reason, if I keep it as HTML, the file will not open in the browser when opened from a Sharepoint directory (mounted in the filesystem or opened through the browser), even though it runs perfectly fine when opened from a local folder. I discovered that simply renaming the HTML file to .aspx allowed it to be opened from Sharepoint. No idea why, I have zero knowledge of .NET and its ecosystem.