[csswg-drafts] [css-border][css-animation][css-transition] Animations/Transitions of border-width after change of computed value definition in issue #11494 (#13511)

weinig has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-border][css-animation][css-transition] Animations/Transitions of border-width after change of computed value definition in issue #11494 ==
In [issue #11494](https://github.com/w3c/csswg-drafts/issues/11494), the computed value (but not the resolved or used value) for the `border-width` properties was changed to no longer take the associated `border-style` into account.

This new definition leads to a change in behavior for animations and transitions of border widths that I don't think is expected. (At least, I think that is the case. There is a lot to the animation specs, so I might have missed something, but it seems like computed values, not resolved or used values, are what are used in making animation decisions).

For example, an animation like:

```css
    @keyframes test {
      from {
        border-width: 3px;
        border-style: none;
      }
      to {
        border-style: solid;
        border-width: 0px;
      }
    }
```

previously would have been a no-op, but now should now cause a 1.5px border to pop-in half way through the animation (when the style transitions from none to solid), which gradually then animates down to 0px. (`3px` and `none` are picked as the initial state, since they are the default values). 

Similarly, a CSS transition like:

```css
    .test {
      transition: 1s ease-in-out;
    }

    .test:hover {
      border-style: solid;
      border-width: 0px;
    }
```

would previously have done nothing, but now will have the 1.5px border pop in half way through.

This has caused at least one regression reported to WebKit, https://bugs.webkit.org/show_bug.cgi?id=307933. (My plan is to use the resolved/used value of `border-width` for checking to see if a transition should trigger, to work around the issue for now. This appears to be what Firefox is doing, though I have not looked at the code to confirm).

My suggestion would be that we specify that `border-width` (and perhaps `outline-width` and `column-rule-width`) uses the resolved/used values for all animation and transition determinations, 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13511 using your GitHub account


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Received on Tuesday, 17 February 2026 18:43:00 UTC