pattern attribute does not validate contenteditable content

Phenomenon

The pattern attribute, which allows regex-based validation on form inputs, does not work on contenteditable regions. Content cannot be validated against a pattern.

Reproduction example

1. Create a contenteditable div with pattern="[0-9]+" (numbers only).
2. Type non-numeric characters.
3. Observe whether validation occurs.

Observed behavior

• In Firefox on Windows, the pattern attribute is ignored on contenteditable.
• No validation occurs.
• Invalid content can be entered freely.

Expected behavior

• The pattern attribute should validate contenteditable content.
• Invalid content should be rejected or flagged.
• Or, there should be a standard way to validate content against patterns.