That should be corrected if anyone invents a time machine. :P
white-space: nowrap should be white-space: no-wrap
animation-iteration-count should just have been animation-count (like column-count!)
vertical-align should not apply to table cells. Instead the CSS3 alignment properties should exist in Level 1.
vertical-align: middle should be text-middle or x-middle because it's not really in the middle, and such a name would better describes what it does.
Percentage heights should be calculated against fill-available rather than being undefined in auto situations.
Table layout should be sane.
Box-sizing should be border-box by default.
background-size with one value should duplicate its value, not default the second one to auto. Ditto translate().
background-position and border-spacing (all 2-axis properties) should take *vertical* first, to match with the 4-direction properties like margin.
Not quite a mistake, because it was a reasonable default for the 90s, but it would be more helpful since then if `background-repeat` defaulted to `no-repeat`.
The 4-value shorthands like margin should go counter-clockwise (so that the inline-start value is before the block-end and inline-end values instead of after them).
z-index should be called z-order or depth and should Just Work on all elements (like it does on flex items).
word-wrap/overflow-wrap should not exist. Instead, overflow-wrap should be a keyword on 'white-space', like nowrap (no-wrap).
The top and bottom margins of a single box should never have been allowed to collapse together automatically as this is the root of all margin-collapsing evil.
Partial collapsing of margins instead of weird rules to handle min/max-heights?
Tables (like other non-blocks, e.g. flex containers) should form pseudo-stacking contexts.
The currentColor keyword should have retained the dash, current-color, as originally specified. Likewise all other color multi-word keyword names.
There should have been a predictable color naming system (like CNS) instead of the arbitrary X11 names which were eventually adopted.
border-radius should have been corner-radius.
Absolutely-positioned replaced elements should stretch when opposite offset properties (e.g. left+right) are set, instead of being start-aligned.
The hyphens property should be called hyphenate. (It's called hyphens because the XSL:FO people objected to hyphenate.)
rgba() and hsla() should not exist, rgb() and hsl() should have gotten an optional fourth parameter instead (and the alpha value should have used the same format as R, G, and B or S and L).
Descendant combinator should have been » and indirect sibling combinator should have been ++, so there's some logical relationships among the selectors' ascii art
The *-blend-mode properties should've just been *-blend
u0001-u00c8.Unicode ranges should not have had a separate microsyntax with their own tokenization or token handling. The tokenization hacks required either to make selectors (e.g., u+a) handle things that are unicode-range tokens, or make unicode-range handle the other huge range of tokens (numbers and dimensions with and without scientific notation, identifiers, +) are both horrible.
font-family should have required the font name to be quoted (like all other values that come from “outside” CSS). The rules for handling unquoted font names make parsing font stupid, as it requires a font-size value for disambiguation. Flexbox should have been less crazy about flex-basis vs width/height. Perhaps: if width/height is auto, use flex-basis; otherwise, stick with width/height as an inflexible size. (This also makes min/max width/height behavior fall out of the generic definition.)
:empty should have been :void, and :empty should select items that contain only white space table-layout: fixed; width: auto should result in a fill-available table with fixed-layout columns.
text-orientation should have had upright as the initial value (given the latest changes to 'writing-mode').
@import rule is required to (a) always hit the network unless you specify cache headers, and (b) construct fresh CSSStyleSheet objects for every import, even if they're identical. It should have had more aggressive URL-based deduping and allowed sharing of stylesheet objects.Selectors have terrible future-proofing. We should have split on top-level commas, and only ignored unknown/invalid segments, not the entire thing.
:link should have had the :any-link semantics all along.
The flex shorthand (and flex-shrink and flex-grow longhands) should accept fr units instead of bare numbers to represent flex fractions.
The display property should be called display-type.
The list-style properties should be called marker-style, and list-item renamed to marked-block or something.
The text-overflow property should always apply, not be dependent on overflow
line-height: <percentage> should compute to the equivalent line-height: <number>, so that it effectively inherits as a percentage not a length
::placeholder should be ::placeholder-text and :placeholder-shown should be :placeholder
overflow: scroll should introduce a stacking context
size should have been a shorthand for width and height instead of an @page property with a different definition
span) with idents in the grid properties, possibly by using functional notation (like span(2)). The alignment properties in Flexbox should have been writing-mode relative, not flex-flow relative, and thus could have reasonably understandable names like align-inline-* and align-block-*.
shape-outside should have had wrap- in the name somehow, as people assume the shape should also clip the content as in clip-path.
It shouldn't be !important — that reads to engineers as “not important”. We should have picked another way to write this.