RFR: JDK-8276800 Fix table headers in NumericShaper.html

Tue Nov 9 10:46:40 UTC 2021

On Mon, 8 Nov 2021 18:25:01 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

>> This change introduces no visual difference, but improves a11y.
>
> Ludvig, could you please elaborate on how this change improves accessibility?

@aivanov-jdk Gladly. [Doccheck](https://openjdk.java.net/projects/code-tools/doccheck/) flagged it, my understanding is that every row needs to have a unique row header to aid screen readers. The previous code had one cell work as header for two rows. 

One could perhaps look at [the standards](https://html.spec.whatwg.org/multipage/tables.html#attr-tdth-headers) and implement something even better, but this is what I came up with.

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PR: https://git.openjdk.java.net/jdk/pull/6291