html - Make div full size of window with NO margin - Stack Overflow

I hate to even ask something that seems so dumb but I just dont get this. All I want to do is have a blue div at the top of my window that stretches across the entire window. I thought width:100%;

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Fix iframe/embedded video size relative to screen size? - Coding - PsychoPy

Overflow · Bootstrap v5.0

css - Can I keep a DIV always on the screen, but not always in a fixed position? - Stack Overflow

Fix mobile viewport 100vh bug in one line of CSS: Dynamic viewport units in action, by Oleksandr Shevchuk

overflow - CSS: Cascading Style Sheets

css - Resizable DIV inside DIV 100% height with margin around not working well! Some help please? - Stack Overflow

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overflow CSS-Tricks - CSS-Tricks