Attention is a budget
Every element on a screen spends a little of the reader's attention. A bold label spends more than a quiet one. A box spends more than a hairline. A second accent color spends more than the first ever did. None of this is wrong on its own, but the budget is finite, and most interfaces overspend it before the reader has found the one thing they came for.
I started thinking about it this way while building a status wall that had to be read from across a room. There was no space for anything decorative. Every pixel that was not the message was noise, and the layout got better each time we removed something.
Three habits
The first habit is to separate with lines, not with boxes. A 1px rule carries almost all the structure a page needs, and it disappears the moment the reader stops needing it. Boxes stay on screen and keep asking to be looked at.
The second is to build hierarchy with weight instead of size. Titles at 15px and 700 next to metadata at 12px and 600 read as two clear tiers, and the page keeps a single quiet rhythm. Once sizes start jumping, every heading competes with the one above it.
The third is to allow exactly one loud moment. On my own site that moment is the hand-sketched name in the hero. Everything after it is restrained on purpose, so the one gesture keeps its charm.
I would rather remove a feature than ship one that shouts.
What quiet is not
Quiet is not empty, and it is not slow. A quiet interface still has to load fast, read well at a glance, and answer the reader's question before they finish asking it. Whitespace that hides the answer is just a different kind of noise.
It is also not a style. Warm paper and thin rules are how I happen to do it. The habit underneath is the same in any palette: spend attention on purpose, and stop spending when the reader has what they need.