HTML: Case Sensitivity

HTML tag names

case-insensitive ✅

but only for English letters, not for other language letters.

For example, if you have a Custom Tag x-ñ and use CSS x-Ñ, it won't work.

• HTML case sensitivity test page (2026-05-22)

HTML attribute name

case-insensitive ✅

but only for English letters, not for other language letters.

For example, if you have a Custom Data Attribute data-ñ and use CSS [data-Ñ], it won't work.

• HTML attribute case sensitivity test page (2026-05-22)

HTML attribute values

case-sensitive 🛑

XML tag name

case-sensitive 🛑

XML attribute name

case-sensitive 🛑

XML attribute values

case-sensitive 🛑