[css-anchor-position-1] `::placeholder` being an acceptable anchor exposes UA implementation details

[dshin-moz]

We're allowing any tree-abiding pseudo-element to be an acceptable anchor. This includes ::placeholder.

However, this exposes an implementation details of <input type=text> - Namely, what happens when the user starts typing into the textbox? Do UAs need to behave as if the box went away? Spec defines ::placeholder loosely enough that ::placeholder having a box while the text is not being shown is fine:

There also exists a :placeholder-shown pseudo-class, which applies to (real) elements while they are showing placeholder text, [...]. ::placeholder specifically represents the placeholder text [...]

position-visibility: anchors-visible being a default value dampens the impact, but anchors-valid would expose implementation details.

[tabatkins]

I'm okay with excepting ::placeholder from that text (it doesn't seem useful to be anchored....)

[fantasai]

@anttijk @nt1m Does this seem reasonable? Are there other form-related pseudo-elements that should be excluded here?