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Warns that attributes were duplicated in one element. Capital letters and lower-case letters are not distinguished.

There must never be two or more attributes on the same start tag whose names are an ASCII case-insensitive match for each other.

Cite: HTML Living Standard Attributes

❌ Examples of incorrect code for this rule

<div data-attr="value" data-Attr="db"></div>

✅ Examples of correct code for this rule

<div data-attr="value" data-Attr2="db"></div>


"attr-duplication": boolean

Default Severity