[whatwg/dom] Can DOM ranges split grapheme clusters and surrogate pairs? (#933)

https://dom.spec.whatwg.org/#ranges

For Text nodes, it seems that the offset of a boundary point is code unit (rather than [grapheme cluster](https://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries)) based, and surrogate pairs might be split.

It would be useful to add a note to remind web developers and specs writers (like [css-highlight-api](https://drafts.csswg.org/css-highlight-api-1/), for example) that grapheme clusters and surrogate pairs might be split, preferably with an example. The note should contain a strong warning against splitting and surrogate pairs.

If possible, DOM should normatively prevent the splitting of surrogate pairs or make it non-conformant.

(This comment is part of a review on behalf of the W3C i18n WG.)


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