| Document Type | application/xhtml+xml | text/html | application/xml | text/xml |
|---|---|---|---|---|
| HTML 4.01 and earlier | MUST NOT | MUST | MUST NOT | MUST NOT |
| HTML 5 (html serialization) | MUST NOT | MUST | MUST NOT | MUST NOT |
| HTML 5 (XML serialization) | MUST | MUST NOT | MUST NOT | MUST NOT |
| XHTML (HTML Compatible) | SHOULD | MAY | MAY | MAY |
| XHTML (not HTML Compatible, or using foreign ns elements) |
SHOULD | SHOULD NOT | MAY | MAY |