A filter list is definitely not HTML
<!DOCTYPE html>
<title>a</title>
Practically, browsers will accept omitting both of these, and the spec even allows for omitting the title "if it is provided by a higher level protocol"So it's not that crazy an argument that a plain text file is a html document
They serve websites to browsers for people to view. This file (be it properly formatted .html or .txt) is not a website people go to in their browser - its used internally by an application. This is the key point.