RFR: 8354791: Use Hashtable.putIfAbsent in CSS constructor [v2]

[Sergey Bylokhov]

On Tue, 29 Apr 2025 13:51:38 GMT, Andrey Turbanov <aturbanov at openjdk.org> wrote:

>> We can use Hashtable.putIfAbsent instead of pair `get`/`put` methods.
>> It's faster and cleaner.
>
> Andrey Turbanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   add CSS. prefix

Marked as reviewed by serb (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/22032#pullrequestreview-2805304709