Setting up first responsive page

[John_Betong]

@coothead

Well, this <div id="containerBody"> , being a block-level
element, has 100% width by default .

I recently gotten my knuckles rapped by assuming the css default width value

Width: 100% and Flexbox

Nope, the default for width is auto. See https://developer.mozilla.org/en-US/docs/Web/CSS/width