To allow customisation, perhaps some kind of remix of the CSS :target and ::selection pseudo-class/element could be an option? (issue #1 seems related but different)

But I’d think that ideally website publishers should not need to customise their site for accessibility: a user agent could just adjust the highlight colour to ensure sufficient contrast; just like some browsers (at least Firefox, but not Chromium I now discover) invert the colours of the user’s selection on dark-ish backgrounds.

Styling the highlight color is under discussion in w3c/csswg-drafts#5233

Regarding accessibility requirements, the browser should provide good defaults. Chrome's reusing the indicating infrastructure that exists around Find-In-Page. I think Chrome could be improved in this case (e.g. crbug/39077. The highlight for text-fragments specifically is currently being debated but this should be implementation defined behavior (certainly warrants some discussion in spec though).

See also issue w3c/csswg-drafts#5522 which is specifically for text fragments.

Update: This has been implemented and is proposed for shipping in blink-dev

The ::target-text pseudo has been shipping for a while now so this is resolved.