Selection.modify() taking optional strings doesn't make sense to me

[emilio]

https://w3c.github.io/selection-api/#dom-selection-modify says:

undefined modify(optional DOMString alter, optional DOMString direction, optional DOMString granularity);

However it also says:

If alter is not ASCII case-insensitive match with "extend" or "move", abort these steps.
If direction is not ASCII case-insensitive match with "forward", "backward", "left", or "right", abort these steps.
If granularity is not ASCII case-insensitive match with "character", "word", "sentence", "line", "paragraph", "lineboundary", "sentenceboundary", "paragraphboundary", "documentboundary", abort these steps.

So, basically, it early returns without errors if any of the arguments don't match. That doesn't make a lot of sense to me.

That seems to match WebKit and Blink's implementation. Gecko however throws a reasonable error and makes the strings mandatory.

I think the Gecko behavior is preferable, but if we're going with the "silent error" behavior of WebKit / Blink, maybe we should clarify it as a compatibility quirk (since it's really odd).

cc @rniwa @smaug---- @foolip

[annevk]

WebKit used to not validate the number of arguments. I suspect this originates from that era.

It also seems like unusual behavior to have ASCII case-insensitive matching for arguments. That makes me a bit less inclined to try and fix it up. Clarifying it and maybe adding a console warning seems reasonable though.

[emilio]

I guess it's fine to match WebKit/Blink then, but it'd be good to point out on the spec that this API behaves weirdly because of compat quirks...

[rniwa]

Yeah, we can add some note to that end.