Of course a missing or extra quotation mark can mess up a whole html page! I've done it and had to depend upon color coding in an editor to find the place where I screwed up. (That last happened in the last milleniam.)
The real killers are things like unclosed <div> or <a> tags. Fun for the whole family.
Interesting about closing tags. When I was learning, we were told to close tags and, as I recall, html pages would not validate without them. Makes me wonder if standards have changed again. I'll probably keep 'em for the sake of "good form" and to be sure where such things end. No reply needed.