From what I recall of that era, it's the exact opposite: invalid elements (like <noscript>) are simply ignored, and their contents shown. That's how <noscript> works: newer browsers which understand JavaScript know the <noscript> element and ignore its contents; older browsers which do not understand JavaScript don't know the <noscript> element so its contents are shown. The same trick is used for <noframes>: browsers like Netscape which understand frames don't show the contents of that element, while other browsers which don't understand <frame> and <frameset> will show the content of the <noframes> element, so it can be used as a fallback.

Strict validation of HTML came later with XHTML, but AFAIK all browsers which understand XHTML also understand JavaScript.