If website developers don’t want browsers to “tamper” with their code in any way, then I say: Go for it. From now on, only perfectly valid websites will be displayed, since leniency toward the rules would in effect change the website.

Headings not in order? “Sorry, this website is broken and can’t be displayed”.

JavaScript error? “Sorry, this website is broken and can’t be displayed.”

Forgot to close a <div>? “Sorry, this website is broken and can’t be displayed.”

An <input> without an associated <label>? “Sorry, this website is broken and can’t be displayed.”

<img> without an <alt> text? “Sorry, this website is broken and can’t be displayed.”

Duplicate IDs on the page? “Sorry, this website is broken and can’t be displayed.”