You are supposed to design your UI based on general principles ("three vertical blocks", "list of elements with a label", "text with paragraphs", etc.) and let

You are supposed to design your UI based on general principles ("three vertical blocks", "list of elements with a label", "text with paragraphs", etc.) and let the user agent lay it out as it wants and accept whatever result happens, and not attempt to test or micromanage it to get a preconceived result. At any rate, HTML layout is determined by the spec barring rare bugs, so you can design something that lays out exactly how you want regardless of browser, except that you must decide and specify the layout for any possible width/height combination.