when box-sizing is not border-box, once width is set for block-level elements It will lose its liquidity

maybe now it is generally required to be responsive. If the width and height are fixed and displayed under each screen, the effect of Taobao is not good. You can take a look at the effect under each screen, and Taobao has a separate style of www.taobao.com for mobile browsers. When the web browser is adjusted to mobile preview mode, the URL of the page will jump directly. Change to https://h5.m.taobao.com/?spre..., if the width and height are not written dead, then the mobile phone web uses a set of code and it looks better

Don't worry too much about what other people say, first of all, how to judge whether there is more or less width.?
and there are many fixed-width units. Vw,vh, and hybrid calc (20vw-10px + 5%), etc.

of course, the most reasonable use is the highest level.
you can learn more about flex layout and grid layout before using these.
use the styles you need where you need them. It's not good to use too much, but it's good to use less. It's the most important thing to be appropriate.