[css-content] content: normal is undefined for the ::placeholder and ::file-selector-button pseudos

https://drafts.csswg.org/css-content/#valdef-content-normal
https://drafts.csswg.org/css-pseudo/#treelike

The "Applies to" field says `content` applies to all "tree-abiding pseudo-elements", but the computed value of `normal` is undefined for tree-abiding pseudos other than `::before`, `::after` and `::marker`.

See also: #4632