Bugzilla

Currently, the XUL `hidden` and `collapsed` attributes are "XUL-style" boolean attributes, which means:
- Only a value of `true` hides or collapses the element in the XUL markup 
- Setting `el.hidden = true` or any truthy value will set `hidden="true"` in the markup
- Setting `el.hidden = false` or any falsy value will remove the `hidden` attribute

It'd be nice to switch to "HTML-style" boolean attributes:
- Setting any value to the `hidden`/`collapsed` HTML attribute will hide the element
- Setting `el.hidden = true` or any truthy value will set `hidden=""` in the markup
- Setting `el.hidden = false` or any falsy value will remove the `hidden` attribute
- The recommended way to write things in HTML markup is `<div hidden>`
- The recommended way to write things in XHTML markup is `<div hidden="hidden">`

The current plan would be:
- Switch the current CSS to read for `[hidden]` rather than `[hidden="true"]`
- Rewrite current JS consumers of `setAttribute("hidden", ...)` to use `toggleAttribute` or `.hidden = ...`
- As bgrins pointed out, some platform code is reading those attributes too: 
  - https://searchfox.org/mozilla-central/source/dom/xul/nsXULElement.h#427
  - https://searchfox.org/mozilla-central/source/widget/cocoa/nsMenuUtilsX.mm#176
- we should be careful about XUL persistance.

Currently, the XUL `hidden` and `collapsed` attributes are "XUL-style" boolean attributes, which means:
- Only a value of `true` hides or collapses the element in the XUL markup 
- Setting `el.hidden = true` or any truthy value will set `hidden="true"` in the markup
- Setting `el.hidden = false` or any falsy value will remove the `hidden` attribute

It'd be nice to switch to "HTML-style" boolean attributes:
- Setting any value to the `hidden`/`collapsed` HTML attribute will hide the element
- Setting `el.hidden = true` or any truthy value will set `hidden=""` in the markup
- Setting `el.hidden = false` or any falsy value will remove the `hidden` attribute
- The recommended way to write things in HTML markup is `<div hidden>`
- The recommended way to write things in XHTML markup is `<div hidden="hidden">`

The current plan would be:
- Switch the current CSS to read for `[hidden]` rather than `[hidden="true"]`, ditto for collapsed
- Rewrite current JS consumers of `setAttribute("hidden", ...)` to use `.hidden = ...`
- Rewrite current JS consumers of `setAttribute("collapsed", ...)` to use `toggleAttribute` or the correct values.
- As bgrins pointed out, some platform code is reading those attributes too: 
  - https://searchfox.org/mozilla-central/source/dom/xul/nsXULElement.h#427
  - https://searchfox.org/mozilla-central/source/widget/cocoa/nsMenuUtilsX.mm#176
- we should be careful about XUL persistance.