CSS classes added when configuring field groups are validated, using too narrow a set of allowed characters. Those of us using frameworks like https://tailwindcss.com/ often use characters that might be unusual, but totally valid. See 🐛 CSS class validation too strict Needs work for another example of this issue elsewhere.
Create a HTML element field group, try using a colon in the class field. The error 'The css class must include only letters, numbers, underscores and dashes.' is shown.
Use a looser validation criteria.
Review patch.
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Needs review
3.0
Code
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My MR#33 is based upon
james.william's patch #3
🐛
CSS class validation too strict
Needs review
. Compared to his patch I've addionally added the four symbols ![]/ to the list of allowed symbols (used for !important, arbitray vaules and group modifiers).