RFR: 8247957: remove doclint support for HTML 4 [v5]

[Jonathan Gibbons]

On Fri, 18 Dec 2020 02:34:16 GMT, Yoshiki Sato <ysatowse at openjdk.org> wrote:

>> HTML4 is no longer supported in javadoc.
>> 
>> doclint needs to drop HTML4 support as well.
>> The changes consist of:
>> * Removing jdk.javadoc.internal.doclint.HtmlVersion and its references.
>> * Sorting out supported tags and attributes in HTML5 (including fix incorrect permission of valign in TH, TR, TD, THEAD and TBODY)
>> * Modifying test code and expected outputs to be checked in HTML5
>
> Yoshiki Sato has updated the pull request incrementally with one additional commit since the last revision:
> 
>   5th: 8258460: Remove --doclint-format option from javac

Looks good.
I'll pull down a copy, for review in an IDE and to build and run tests, and if that goes OK, I will approve it.

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PR: https://git.openjdk.java.net/jdk/pull/893