A couple of questions about the ways of vertsky?

Question

css

Hello, I just started to type. While typesetting, there were many questions that I did not know how to answer and there was no one to ask. I use gulp+jade+stylus and work in brackets. Css and html I know probably, but weakly in my opinion. Js I do not know, at least.
Here is a list of questions:
1. Sometimes I see

<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">

such a meta tag, why is it? How important is it to use it?
2. Modernizr - I seem to have read what it is, but how is it used in practice?
3. First I used grid layout (no framework), then I switched to flex. Flex is very convenient, but there are many bugs in different browsers. At first, everything is beautiful, but I am tormented by how to treat bugs here and there in different browsers. Thank God fixing them is easy. What is the best markup to take? Or what's more convenient? Does it make sense to use grid layout + flex? Those. make a grid, and use flex inside the grid?
4. How to make the first block on the whole page for different sizes? Well, I read that html, body and the block itself must be made height 100%, but it still didn’t work out normally.
5. Vmin etc. seems to be comfortable. to create responsive sites, but the aspect ratio is still not preserved. I'm using rem at the moment, but not everything is cool with it either. Maybe there are articles that will explain which units are better. use when creating a responsive website.
6. I often see in css markups

/*-------------------------------
   Test
--------------------------------*/

Are the developers writing this manually or is there a plug-in that immediately draws like this? commenting blocks write them manually or are there plug-ins?
And I also see maps in css markups. What is the best way to do markup in general, is there an article that explains where it is better to start writing css?
7. Are there any plugins that allow you to turn js on and off on ODA. screen resolution?
8. Does it make sense to take a steam bath for html5 tags article, section and the like? Or further to build stupidly all through div?
9. Now I'm suffering with one bug, the blocks are collapsing. On all major browsers FF, Opera, Chrome, IE, Edge is perfectly displayed, but on the android 5.0 browser the blocks collapse and how to determine why this is without the developer console?
10. Recommend an online showcase for goods, in which there will be a lot of goods, but you can’t buy them. But the data must be dynamic. It seems to me that AngularJs would have coped with this, but there are few ready-made scripts on it.
11. In html5up.net templates in css I see such selectors .container.\37 5\25 - what is it?
12. Is there a way to build a layout from the mobile version if there is no mobile version layout? xD
PS To be honest, there were a lot of questions, but I didn’t write them down and in the end it turned out that I sucked the questions out of my finger
Thank you in advance.