light-dark()
Two color values in one declaration, picked by the color scheme, which removes most of the reason a theme needed a second block of custom properties at all.
scrollbar-gutter
The layout shifts by fifteen pixels when a modal opens, everybody blames the modal, and the actual cause is the scrollbar disappearing along with it.
text-wrap: balance and pretty
Two values that fix the two ugliest things typesetting does on the web, and the rule for which to use is entirely about how many lines you have.
Intl.NumberFormat
Currency formatting is in the browser, has been for years, and is still being done with a hardcoded dollar sign and a call to toFixed in most of the code I read.
AbortController
The thing everybody knows as the way to cancel a fetch is also the tidiest way to remove a pile of event listeners, and the second use has changed more of my code than the first.
:is() and :where()
Two selectors that look interchangeable in every example anybody writes, and differ on the one thing that actually causes problems later.
details and summary
An accordion with keyboard support, correct announcement and no JavaScript at all, which most of us reimplemented by hand for about a decade before noticing.
prefers-reduced-motion
An operating system setting that has been sitting there since 2013 telling you somebody gets motion sick, and the two-line stylesheet that is the whole of respecting it.
git reflog
Almost nothing is actually gone after a bad reset, because git keeps a local log of everywhere HEAD has pointed for the last ninety days and hardly anybody knows to look at it.
fieldset and legend
A group of radio buttons has a question attached to it, and the question needs to be in the markup rather than in a heading that happens to sit nearby.
word-break and overflow-wrap
Two properties that sound like synonyms, do different things, and get reached for in the wrong order roughly every time a long URL breaks out of a card.
git bisect
I bisected a broken stylesheet by hand last week, through a pile of backup copies on a live server, and it made a better case for version control than anything I've said out loud.
optgroup
A select with 40 options in it is nearly always a select that wanted three or four labeled groups, and the element for that has been sitting in HTML the entire time.
abbr and the title attribute
The abbreviation element is one of about four places in HTML where the title attribute is the actual content rather than a decoration, and it is still unreachable for anybody not holding a mouse.