RFR: JDK-8288531: Empty spans in mobile navigation markup

[Hannes Wallnöfer]

The empty spans in the mobile navigation button are removed with a warning by HTML Tidy in the default setting.

I tried three different solutions:

 - Add a normal space character: this did not satisfy HTML Tidy, it still removed the spans with a warning.
 - Add an HTML content: this stopped HTML Tidy from removing the spans, but the three comments added a lot of visual noise.
 - Add a ` ` entity: this stops HTML Tidy from removing the spans, doesn't add too much to the page and does not affect the layout. 

I went with the third option after making sure the layout is not affected (tested on Chrome, Firefox, Safari on macOS as well as Safari on iOS and Chrome on Android). Behaviour of HTML Tidy was tested with version 5.8.0 on macOS.

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Commit messages:
 - JDK-8288531: Add comment
 - JDK-8288531: Empty spans in mobile navigation markup

Changes: https://git.openjdk.org/jdk19/pull/53/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=53&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288531
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk19/pull/53.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/53/head:pull/53

PR: https://git.openjdk.org/jdk19/pull/53