[cssom] Clarification to the "6 decimals" of serialized CSS number values

[wangxianzhu]

https://drafts.csswg.org/cssom/#serializing-css-values says the following for serialized CSS number values:

... rounding the value if necessary to not produce more than 6 decimals ...

It is not clear if the wording of '6 decimals' means '6 significant digits' or '6 digits after the decimal point'. Current browsers seem to all implement the former.

It'll be better to use more precise wording as 'decimal' is ambiguous.

[dbaron]

(This came from discussion in crrev.com/c/3722705.)

[cdoublev]

Also have a look at #6471 (comment).