fix: extend and clarify valid tag names

[denis-anisimov]

fixes #9669

[vaadin-bot]

This ticket/PR has been released with platform 20.0.0.alpha8. For prerelease versions, it will be included in its final version.

[vaadin-bot]

Hi @denis-anisimov , this commit cannot be picked to 6.0 by this bot, can you take a look and pick it manually?
Error Message: Error: Command failed: git cherry-pick 76ce842
The previous cherry-pick is now empty, possibly due to conflict resolution.
If you wish to commit it anyway, use:

git commit --allow-empty

Otherwise, please use 'git reset'

[vaadin-bot]

Hi @denis-anisimov , this commit cannot be picked to 2.6 by this bot, can you take a look and pick it manually?
Error Message: Error: Command failed: git cherry-pick 76ce842
The previous cherry-pick is now empty, possibly due to conflict resolution.
If you wish to commit it anyway, use:

git commit --allow-empty

Otherwise, please use 'git reset'

[vaadin-bot]

Hi @denis-anisimov , this commit cannot be picked to 2.5 by this bot, can you take a look and pick it manually?
Error Message: Error: Command failed: git cherry-pick 76ce842
The previous cherry-pick is now empty, possibly due to conflict resolution.
If you wish to commit it anyway, use:

git commit --allow-empty

Otherwise, please use 'git reset'