Q:

On what the margin impact

I was thinking, about how the margin influence other objects?
Because when I have e.g.
html, body{
    height: 100%;
    width: 100%;
}
div{
    margin-top: 200px;
}

and inside I have div with margin-top it push <body> down, so the margin impact on <html> not on <body> tag, which is first container.
So the question is, how margin depend on

parent tag
siblings tags
floating, absolute, relative or static elements

I have readed few css tutorials, but I couldnt find specific answer

A:

Most browsers by default add a margin to the <body> element. One thing you haven't done in your styling is to reset this to 0:
html, body{
    height: 100%;
    width: 100%;
    margin: 0;
}

Why this happens is a question that has been asked many times here. See: What is the point of CSS collapsing margins? and Why does this CSS margin-top style not work?.

