What problem does this feature solve?

Currently if you pass a hash id with unescaped CSS special characters /[!"#$%&'()*+,\-./:;<=>?@[\\]^{|}~]/ to a scrollBehavior selector, execution will fail:

'XXX' is not a valid selector

Because of this url-styled hashes (e.g. "#one/two" or "#main/secondary"), both valid ids in HTML5, cannot be passed directly from the to.hash or from.hash parameters.
This is because behind the scenes document.querySelector, which relies on CSS character compliance, is used in all but a very small number of circumstances (i.e. /^#\d/). This feature proposes broadening the use case for document.getElementById to all selectors that have the appearance of an id, specifically ungrouped selectors starting with a hash (i.e. /^#[^, ]+$/)

What does the proposed API look like?

API doesn't change, except the following would work:

scrollBehavior(to, from, savedPosition) {
  return {
    selector: to.hash,
  }
}
// where:
window.location = '/one/two#three/four';