I didn't find any statement on default values for style properties of ::highlight in the spec draft. There are some default values in the spec for pseudos but ::highlight is not in the list. Also in the spec draft of ::highlight there's no mention of it.

Should it use no visual effects or should it use for example Highlight/HighlightText colors instead? I assume it's taking no visual effects as default values (as in when background-color is not specified inside a ::highlight(foo) then the content's background is the same as if there was no ::highlight defined), but maybe it should be explicitly stated?