> there is no benefit for the user if it fails to to render some existing pages

What if the browser renders it incorrectly? If a corrupt tag combination leads to browser X parsing "<script>" as inline text but browser Y parsing it as a script tag, that could lead to serious security issues!

Blindly guessing at the original author's intent whenever you encounter buggy content is a recipe for disaster. Sometimes it is to the user's benefit to just refuse to render it.