Is it really true to begin with? I mean, yes, naively setting height: 50% vs. setting width: 50% will do different things but that's not because height and widt

Is it really true to begin with? I mean, yes, naively setting height: 50% vs. setting width: 50% will do different things but that's not because height and width are doing anything differently fundamentally[0], as OP claims[1], but because the containing block's (and their containing block's etc.) default values for width/height differ: By default, block elements grow in height (fit their content) but take up a width of 100%, since that seems sensible for documents as the sibling comments point out. Now the issue is that if you set height: 100% somewhere down the tree, it refers to the height of the containing block. But if the latter wants to adapt to the height of its content, it's catch 22. The height: 100% won't have any effect. [0]: MDN says the following about setting a percentage for height/width: > <percentage>: Defines the height/width as a percentage of the containing block's height/width. [1]: See also https://geoffgraham.me/width-looks-outward-height-looks-inward/ https://geoffgraham.me/width-looks-outward-height-looks-inwa... where OP points to the YouTube video he got that line from.