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%;

overflow CSS-Tricks - CSS-Tricks

正規店または公式サイト 【COLOR iN ID】check noob padding

What is Margin Collapse in CSS? And How to Avoid It

html - Make a div take up the full space not used by margins - Stack Overflow

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html - Div width 100% minus fixed amount of pixels - Stack Overflow

Make A Div Fill The Height of The Remaining Screen Space, PDF, Html Element

CSS Height Full Page: CSS gotcha: How to fill page with a div? - DEV Community

CSS Height Full Page: CSS gotcha: How to fill page with a div? - DEV Community

html - Css width:fit-content; property doesn't work well. Parent element has this property calculates its width smaller than its content - Stack Overflow

overflow CSS-Tricks - CSS-Tricks

DivWindow - CodeProject

Why margin on child element moves the parent element in CSS ? - GeeksforGeeks