Many WPT tests in the custom-elements/reactions tree fail in a characteristic way that points at a common cause.

In custom-elements/reactions/HTMLButtonElement, a custom element is made that derives from HTMLButtonElement, and its .autofocus is assigned to, using instance[jsAttributeName]=... syntax as opposed to instance.setAttribute(...). No attributeChanged event fires. For HTMLButtonElement, it's specifically autofocus that fails like this; for other elements it's various other specific attributes. Searching in test output for "must enqueue an attributeChanged reaction" seems to list them all:

Element

• slot

ElementContentEditable

• contentEditable

HTMLButtonElement

• autofocus

HTMLAreaElement

• rel
• referrerPolicy
• shape
• alt
• download
• ping
• coords

HTMLBaseElement

• target

HTMLElement

• tabIndex
• draggable
• translate
• spellcheck
• dir
• accessKey

HTMLEmbedElement

• src
• type
• height
• width
• align
• name

HTMLFieldSetElement

• name

HTMLImageElement

• lowsrc
• referrerPolicy
• sizes

HTMLMapElement

• name

HTMLMeterElement

• max
• value
• high
• low
• min
• optimum

HTMLOListElement

• reversed
• type
• start
• compact

HTMLMetaElement

• httpEquiv

HTMLOptGroupElement

• label

HTMLModElement

• dateTime
• cite

HTMLParamElement

• name
• value
• type
• valueType

HTMLProgressElement

• value
• max

HTMLQuoteElement

• cite

HTMLStyleElement

• media

HTMLTableCellElement

• headers
• scope
• abbr

HTMLTableColElement

• span

HTMLOutputElement, HTMLSelectElement, and HTMLTableElement are getting slightly different failures I'll look at separately.