| Error Name | Explanation |
|---|---|
| html-outer | outer tags should be <HTML> .. </HTML> |
| no-head | missing <HEAD> |
| head-element | heading-only tag (TITLE, NEXTID, LINK, BASE, META) found outside of heading |
| no-body | missing <BODY> |
| must-follow | required tag does not immediately follow another |
| unclosed-element | unclosed elements (e.g., <H1> ... ) |
| netscape-markup | Netscape-specific tag |
| empty-container | empty container element |
| mis-match | mis-matched tag (e.g., <H1> ... </H2>) |
| heading-order | order of headings (e.g., <H3> following <H1>) |
| netscape-attribute | Netscape-specific attribute |
| unknown-element | unknown tag |
| body-no-head | <BODY> but no <HEAD> |
| unknown-attribute | unknown attribute |
| no-title | missing <TITLE> |
| element-overlap | overlapped elements |
| required-context | failed context check (where a tag must appear within a certain element) |
| nested-element | illegally nested element |
| odd-quotes | odd number of quotes in tag (unclosed quotes) |
| non-head-element | tag other than a valid heading tag (ISINDEX, TITLE, NEXTID, LINK, BASE, META, RANGE, STYLE) found in heading |
| once-only | catches elements which should only appear once |
| unexpected-open | unexpected < (potentially unclosed element) |
| closing-attribute | closing tag should not have any attributes specified |
| require-head | expects to see a TITLE in the HEAD element |
| illegal-closing | element is not a container |
| repeated-attribute | repeated attribute |
| expected-attribute | missing expected attribute |
| unclosed-comment | unclosed comment |
| obsolete | obsolete element |
| markup-in-comment | markup embedded in comments (can confuse some browsers) |
| required-attribute | missing required attribute |
| leading-whitespace | should not have whitespace between < and tag |
| attribute-delimiter | use of ' for attribute value delimiter is not supported by all browsers |
| directory-index | directory does not have an index file |
| literal-metacharacter | use of a literal metacharacters instead of an &-symbol |