You really can't place the blame for there being multiple "versions" of the web on parsers being forgiving. That has more to do with an incomplete/incorrect implementation of the HTML specification.
Sure I can, as soon as one browser decides it's going to be "forgiving" with something non standard, then some users will start to code in that way. This means we now have another version of the web that will not work in anything but the stupid browser that decided it would be "forgiving".
And, as web pages are a very visually-focused medium, I'd rather we get the display problems worked out before we move on to making sure we can have neat spinning bars and boxes that jump out of the page at you.
I'm not talking about spinning bars, I'm talking about Gmails, youtubes, krugles, local maps and message boards just like this one which have been DRASTICALLY held back by users who don't realise how much it is affecting them and the rest of the world. Using a non standard browser is the tech equivilient of not signing the Kyoto Protocol. Punishing the future simply because the present is comfortable.
So, getting back to the discussion, I'm glad there's a certain level of forgiveness when I write a page. If I forget to close my image tag when I write a quick blog post, I'd rather the page show up fine than completely "crashing." If some guy messes up when he writes a link in my comment box, I don't want that to "crash" my page either. If the browser can figure out what he was trying to link to, then it's even better.
All browsers would guess differently for every little forgiveness. However instead they could all not parse your page and just tell you that you didn't close the image tag. You close it and then it displays the same in every browser.
The web is a place that changes very quickly, very often and I think that uber-strict parsers would hinder that.
If you only knew how much things are currently hindered. If only! One day in the shoes of a developer working on something slightly advanced and you would be a convert for life and a standards zealot.