We need lightweight HTML/JavaScript applications that aren't an entire web browser. You already have a web browser installed, you shouldn't need yet another copy of Chromium to run some HTML/JS files. Because browser makers cripple HTML files run from your own computer (can't fetch any files, even from the same directory), you are forced to start a web server just to run HTML and JS files from your own hard drive.

If you have python installed, you can start a web server with a couple lines of shell script. Is there any really good truly lightweight webserver? (Something under 256KB)

No we don't. Browsers are wrong abstraction for GUIs