That should be corrected if anyone invents a time machine. :P

white-space: nowrap should be white-space: no-wrap and line wrapping behavior should not have been added to white-space

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-start value).

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

The syntax of unicode ranges should have consistent with the rest of CSS , like 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 FIXED in the spec , waiting for implementations to check for Web-compat…

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').

The @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(2) ). We probably should have avoided mixing keywords (span) with idents in the grid properties , possibly by using functional notation (like).

comments shouldn't have been allowed basically everywhere in CSS (compare to HTML , which basically only allows them where content goes), because it makes them basically unrepresentable in the object model, which in turn makes building editing directly on top of the object model impossible

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.