Browser error and strange reaction to CSS change, how to solve?

Question

HTML

Hello.
I have a problem and I don't understand what's going on. Something broke in the layout or problems with the browser. I did everything according to the tutorial video. Everything worked. brackets editor. Further, in order to clean up the code a bit, I sent it to VSC and walked through it as a pretier, put it back into brackets and the browser began to work strangely.
An error appeared in the developer panel: Failed to load resource: the server responded with a status of 404 (Not Found)
and Cannot GET /favicon.ico .
At the same time, the page in live view reacts strangely to changes in the code in css, namely, it perceives individual edits, but not others. I delete entire markup blocks, in theory the blocks should disappear, but they remain in their original places.
I change the margin, the blocks diverge, I completely remove the margin - the original 15px remains. Tell me where to dig and why is this happening?