Customizable `<select>` element

[josepharhar]

This proposal includes parser, styling, and rendering changes to make the <select> element fully customizable by authors.

Explainer: https://open-ui.org/components/customizableselect

Spec PR: #10548

Spec PRs (old):

• Relax <select> parser #10557
• Update content model for customizable select #10586
• Define base appearance for <select> #10629
• Define the <selectedcontent> element #10633
• Add UA stylesheet for <select> base appearance #10670

CSS spec PRs:

• [css-ui-4] Define appearance:base w3c/csswg-drafts#10691
• [css-pseudo-4] Define ::picker() w3c/csswg-drafts#10865
• [css-pseudo-4] Fix grammar for ::picker w3c/csswg-drafts#10936
• [css-forms-1] Add new pseudo-elements for customizable select w3c/csswg-drafts#10986

Accessibility PRs:

• Update: add mapping tables for customizable select w3c/aria#2369
• [html-aam] Addition: selectedcontent element w3c/aria#2344
• Add selectedcontent element and allowances for 'button' part of select w3c/html-aria#528
• Update: legend to provide name to optgroup w3c/aria#2360

Related HTML issues:

• Timing of cloning for the <selectedoption> element #10520
• Should datalist, option, object, and/or iframe elements be listed in "interactive content"? #10755
• How to spec user interaction for select #10762
• Is it ok for appearance:base <select> not to require user activation before showPicker()? #10604
• Selection in form control elements, user-select, and customizable select #10876
• Content model and 'what' to render for stylable <select> elements #10317
• HTML parser changes for customizable <select> #10310

Related CSS issues:

• [css-ui] Changing UA styles based on the computed value of the appearance property w3c/csswg-drafts#10028
• How to implement and shape API for <selectedoption> element for <select> w3c/csswg-drafts#10242
• [css-ui] New value(s) for the appearance property to support <select> stylability w3c/csswg-drafts#10333
• [css-ui] DOM/Box structure for appearance:base-select w3c/csswg-drafts#10380
• [css-ui] Pseudo-elements for stylable select w3c/csswg-drafts#10462
• [css-ui] Pseudo-element for select's UA button w3c/csswg-drafts#10717
• [css-ui] Pseudo-element for select's UA popover w3c/csswg-drafts#10758
• [css-forms-1] Should ::picker(select) match :popover-open? w3c/csswg-drafts#10775
• [css-ui] UA stylesheet for appearance:base <select> w3c/csswg-drafts#10857
• [css-ui] Pseudo-elements for checkmark and dropdown icon for appearance:base <select> w3c/csswg-drafts#10908
• [css-ui] Colors to use for appearance:base <select> w3c/csswg-drafts#10909
• [selectors-4] Should we have :open and :closed? w3c/csswg-drafts#11039
• [css-ui] select:hover and select:active styles w3c/csswg-drafts#11185

Standards positions:

• HTML selectedcontent element mozilla/standards-positions#1142
• Customizable select element mozilla/standards-positions#1060
• Relax <select> parser WebKit/standards-positions#414
• Relax <select> parser mozilla/standards-positions#1086
• Customizable select element WebKit/standards-positions#386

[zcorpan]

@whatwg/forms

[rohankaushal123]

I'd like to work on this issue @zcorpan

[annevk]

Thank you for bringing this proposal to the WHATWG Joey! I thought this would be a good opportunity to outline how colleagues and I feel about extending HTML in this area. In particular, we feel that new and existing form controls:

• Should look the same as the operating system controls by default.
• Should be fully styleable by web developers.
• Should generally attempt to follow existing HTML element patterns.
• Should not be redundant with existing HTML form controls.
• Should work on a wide variety of platforms, including those with very small screens, no mouse or touch, etc.
• Should be fully accessible.
• Should not have any l10n or i18n shortcomings.

We understand that the select element can’t address a variety of scenarios due to parser limitations, but the select element could address them in combination with the datalist element. One of our big worries with complete duplication is that we end up not solving the problems with the existing controls and that the duplicated controls will have a variety of shortcomings the older controls did not have.

(Aside: #5791 seems very related to this issue.)

[hsivonen]

I think the explainer doesn't explain the need for a new element in a convincing way, because the multiple examples look addressable in the context of select: flags are addressable by emoji instead of image files, subheadings by the label attribute of optgroup, and color and font-level styling seems feasible without a new element.

(From off-explainer examples, I gather that the ambition level of the feature is much higher than the explainer makes appear.)

[josepharhar]

Thanks for taking a look and discussing with colleagues Anne!

Should look the same as the operating system controls by default.

If we made appearance:none or a new appearance value like appearance:base as discussed here and here have a standardized style, then the default appearance:auto appearance could look the same as OS controls. That would be up to each UA to decide, as it is currently for all other form controls.

Should be fully styleable by web developers.

Agreed! Styleability is the primary driving use case for the current proposal.

Should generally attempt to follow existing HTML element patterns.

Agreed! Domenic left some feedback in this vein and we completely revamped the proposal to use existing HTML element patterns by using new tag names instead of slot and behavior attributes.

Should work on a wide variety of platforms, including those with very small screens, no mouse or touch, etc.

Agreed! Existing elements conform to this by letting the browser/OS provide a native picker which unfortunately can’t be styled or customized by web developers, right? It was recommended that we let the browser use native pickers for selectlist on mobile here. Ideally developers would be enabled to make their own pickers that work on these platforms when they want to as well.

Should be fully accessible.

Agreed! This is also a core requirement for the proposal. I have been working closely with accessibility experts participating in OpenUI to ensure that the accessibility mappings and keyboard behaviors follow the current best practices in ARIA.

Should not have any l10n or i18n shortcomings.

Agreed! The content is fully customizable/replaceable, so it should be localizable and internationalizable, right? The only thing I can think of is supporting right-to-left and vertical writing modes when rendering where the listbox popup goes, which is something we are doing for other form control elements and we can easily do via Anchor Positioning for selectlist. Is there anything else you had in mind here?

Should not be redundant with existing HTML form controls.

We understand that the select element can’t address a variety of scenarios due to parser limitations, but the select element could address them in combination with the datalist element

It sounds like yall would prefer to reuse the existing <select> element instead of creating a new tag name. Am I correct? As you mentioned, we can’t have the new content model exist inside the <select> element due to parser rules, but we could try to do something with how datalist works by adding attributes to <select> which refer to the other elements we want to use, which would perhaps look like <select list=my-listbox button=my-button>. However, I don’t think this would work well. This works for <input list=datalist> because datalists are just used to host text data without doing actual rendering. Since we are actually trying to render an authored <listbox> or <button> element inside the select/selectlist, we don’t have the machinery in the browser to render them from an arbitrary location in the DOM. ShadowDOM is perfect for this and is how existing builtin elements like details and summary work. It is also easier and more idiomatic (like <option> and <summary>) for the new listbox and button elements to know when they should have special behavior for selectlist because they can look at their parent context rather than having to keep track of which select element has an attribute pointing at them. A preview of the State of HTML survey I’ve seen also lists <datalist> as a top developer pain point, which perhaps alludes that the ergonomics of referring to a separate element aren’t great.

One of our big worries with complete duplication is that we end up not solving the problems with the existing controls and that the duplicated controls will have a variety of shortcomings the older controls did not have.

I agree that duplication should be avoided. Selectlist is not a complete duplicate of select. <select> can draw outside the bounds of the browser window, and <selectlist> has a completely different rendering model that allows full customization. If it were possible to shoehorn into <select> that would be nice, but it looks infeasible.

[josepharhar]

I think the explainer doesn't explain the need for a new element in a convincing way, because the multiple examples look addressable in the context of select: flags are addressable by emoji instead of image files, subheadings by the label attribute of optgroup, and color and font-level styling seems feasible without a new element.

Thanks for the feedback! I am improving the examples here: openui/open-ui#918

[lukewarlow]

flags are addressable by emoji instead of image files

I just wanted to point out Windows doesn't provide flag emojis so this specific example is a good use case for needing images not emoji.

[annevk]

Existing elements conform to this by letting the browser/OS provide a native picker which unfortunately can’t be styled or customized by web developers, right?

If you use appearance:none they can usually be styled. And we should do more work here to add something akin to appearance:base to make that more ergonomic for web developers. I don't think the solution should be adding new elements that don't have platform theming by default as that would make the HTML language inconsistent.

ShadowDOM is perfect for this and is how existing builtin elements like details and summary work.

For <datalist id=options> we'd have to clone the options into the select element's shadow tree for rendering purposes, I think, much in the same way we probably have to clone one of the options as the selected option. As long as this happens in the internal shadow tree it's largely not observable and should be okay.

It would also have the benefit of having a consistent design with <input type=text list=options>, so we'd tackle combo boxes at the same time (or at least make it possible to reuse the design for them). Since the <datalist> continues to mainly serve as a source of data I don't think there's any need to track what element is pointing to it.

(There are some challenges here with how to match these options and their elements from CSS, but that seems like a surmountable problem.)

I haven't looked at survey in detail, but I'd assume that if <datalist> is mentioned as a pain point it would mainly be around not being able to style it, which is something we'd have to tackle for combo boxes and select alike.

[josepharhar]

I’ve been discussing with @annevk and @pxlcoder about how to move forward here, and we have reached agreement on some things. Please correct if I misrepresent anything!

• Reusing the <select> element will work
• We can change the parser for <select> to allow particular new child tags like <button> and <datalist>
• We can work incrementally, first by making these parser changes in the spec etc.
• Using <datalist> as a child of <select> will work to replace the listbox with custom content

[lukewarlow]

I'm concerned by the implications of the multiple attribute. How does it work with the new parsing?

That being said I am excited by the progressive enhancement possibilities.

[gregwhitworth]

If you use appearance:none they can usually be styled. And we should do more work here to add something akin to appearance:base to make that more ergonomic for web developers.

@annevk The majority you cannot and it isn't solely about styling but also extending them. While select may be able to be re-used; one of the benefits of introducing the new elements that are interoperable by default and not OS platform styled does make the platform consistent rather than trying to retro-fit all the build-in UI controls. I think we may want to fork this into a more generic issue specific to the introduction of new elements as I'm curious how your assertion holds up for input type=switch that not only allows styling but extensibility without a new element; type=file, type=progress, etc?

Using <datalist> as a child of <select> will work to replace the listbox with custom content
This makes sense to me outside of my position that we should introduce a new element, not a new CSS attribute to solve this problem as it's not just about styling

@josepharhar @annevk so will this likewise have the child of <selectedoption> and ability to modify the initial shadow DOM?