abrodski, you have not yet said--in this thread or the other one--whether these messages are popping up on your own pages or on random other pages you’re browsing. If it concerns other people’s pages, the question may get a more useful answer in the Edge and MSIE subforum [webmasterworld.com]. If it is about your own pages, use a validator for more information.
I'll guess that the page(s) in question are your own. To determine whether there are errors, you can use the HTML Validator [validator.w3.org] to check the HTML and tell you if there are items you can fix. Generally they offer a report that lists any errors and how to fix them.
not2easy, Browsers can't do stuff by themselves yet. Maybe except for annoying push-messages :-) I myself did press F12 buttom to call the Dev Tools of the FF. THERE in "Console" section I'd seen what I told you I had.
lucy24, My question here and there is very simple, actually. Not how to correct those issues, but do I have to. If those issues aren't critical for my site's well-being and its proper functioning, then I won't bother. Since I'm not a coder it's not so easy for me just to fix them.
Not how to correct those issues, but do I have to.
There is really know way for anyone to answer this question without knowing the specifics of the page. The browser is most likely rendering the page despite the error and everything is fine. But there may be some unexpected impacts that you are not seeing. Ideally one would want to correct the error.
Given that you are experiencing both errors together, it would suggest to me that there is an issue with the closing body tag. Possibly appearing twice in the document. I would run the page through the W3 validator, find the problem. Then it should be easily fixed.
Looking up these errors (also the other 1509 post) on the MS website/forums ... these are ordinary errors for webpage failure to meet strict html tags, going back to IE9 through the latest iteration of IE and also found in Edge as well. MOST appear to be a result of using <p> incorrectly, but not always.