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First stab at #289 #501

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First stab at #289
LeaVerou Jul 15, 2024
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LeaVerou Jul 15, 2024
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LeaVerou Jul 17, 2024
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Expand Up @@ -1032,6 +1032,25 @@ The alternative user experience is a flash of unstyled content, which is undesir

See also [[#worker-only]].

<h3 id="html-attribute-property-reflection">Expose content attributes for writable IDL attributes</h3>

When designing a new *writable* IDL attribute to specify element state,
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The emphasis on "writable" doesn't seem to serve a purpose. (I, for one, find it distracting.)

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I disagree, I think "writable" is essential to the principle and should be emphasized.

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I would suggest a rephrase then.

When designing a new IDL attribute that reflects the state of an element,
expose a corresponding content attribute,
unless the attribute is immutable or
there are other strong reasons not to.

expose a corresponding content attribute unless you have a compelling reason not to.

Providing declarative ways to manipulate state has several benefits for authors:
- It takes advantage of HTML’s [reactivity](https://en.wikipedia.org/wiki/Reactive_programming), and alleviates the need to apply changes at a specific moment in time.
- It integrates with existing DOM methods, improving learnability, whereas an API specific to the feature needs to be learned separately.
- Behaviors can be expressed reactively in libraries supporting HTML-based syntax and be handled by any library handling HTML generically.
- UI states can be addressed in CSS via simple attribute selectors rather than new pseudo-classes.
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I wonder if this is a good thing in all cases. For instance, [lang=foo] v. [lang|=foo] v. :lang(foo).

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This seems orthogonal. In that case the pseudo-class serves a distinct purpose by targeting the actual computed language (which may be specified on an ancestor). I’d argue there may even be value in a generic attribute selector for these inherited attributes.

- State can be inspected and manipulated via the browser's developer tools with no need for implementing a new UI.

<div class="example">
A counterpattern to this guidance can be found in
<{input}>'s {{HTMLInputElement/indeterminate}} IDL property
which was never exposed as a content attribute making it impossible to fully update checkbox state
by modifying HTML.
</div>
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On the other hand, indeterminate is a terrible feature that should never have been added to the platform in the first place. Maybe we should try to find a more positive example?

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Could you elaborate? Is it the UI of an indeterminate checkbox that you think is a mistake or the way it was added to HTML?

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All of the above, IIRC.

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@annevk annevk Jul 17, 2024 •
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The indeterminate IDL attribute is a bad example as it's like the checked or value IDL attributes which also do not have corresponding content attributes.

You could argue that we should have had a defaultIndeterminate IDL attribute for a corresponding indeterminate content attribute, but that's a separate matter. whatwg/html#6578 goes into this a bit.

(And of course people always like to point out that the IDL attributes not matching the names of the content attributes is confusing, but that's also a separate matter and anyway not something that can actually be solved.)

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@LeaVerou LeaVerou Jul 17, 2024 •
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There’s a separate, existing, principle about this: Keep attributes in sync. It’s right below this one actually 😁

indeterminate in addition to that, also has no content attribute counterpart so there is no HTML way to specify it at all (current OR initial status).

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@hober Interesting. Could you elaborate on the UI part?

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@annevk annevk Aug 21, 2024 •
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That principle is about content attributes needing to be reflected right? But indeterminate is not a content attribute. Edit: ah right, it even says so in the note:

This does not hold the other way around. A new IDL attribute does not always warrant a content attribute counterpart.


<h3 id="html-idl-must-by-synced">Keep attributes in sync</h3>

New content attributes
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