I’m not saying it’s the answer, but it’s a starting point. I’m guessing you have an index.php in your theme. You need to go through it with a fine toothcomb making sure that every <div …. is matched by a </div>
In fact, the very first warning about the </p> might be the starting point ??
What I would do is install HTML validator in Firefox, or paste the source into https://validator.w3.org/#validate_by_input to see what progress you’re making. I’d probably get a copy of the source, paste it into a text editor, save as an HTML file, remove all images, remove all blocks of text, until I’d stripped it down to the elements, every now and again checking to see what it looked like in IE.
Then I’d remove elements one block at a time e.g. <p> </p>, hopefully finding if there’s any that are missing.
That’s just a general approach. It just takes time. If you’ve done it enough times it only takes five to ten minutes (if all goes well) ??
It’s like finding a problem on a car – eliminate one thing at a time – it’s the quickest way.