Sure, authoring tools should help authors avoid mistakes and produce valid content. But the browser is a tool for the consumer of content, and there is no benefit for the user if it fails to to render some existing pages.
It is like Windows jumping through hoops to support backwards compatibility even with buggy software. The interest of the customer is that the software runs.
> Windows jumping through hoops to support backwards compatibility even with buggy software
This was, maybe, true some 10 years ago. Now even old Windows programs (paint,wordpad) do not run on newer Windows
> The interest of the customer is that the software runs
Yes, but testing is expensive and we are Agile. /s
if developer accidentally left opening comment at the start of the html.
Rhetorical question: Should the browser display page even if it is commented out?
There is some bar for what is expected to work.
If all browsers would consistently error out on unclosed tags, then it would definitely force developers to close tags, it would force it become common knowledge, second nature.
> 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.