Apart from purely syntactical errors, many things can go wrong in html, especially with respect to hyperlinks, anchors and id’s — as those are often manually maintained.
Primary sources of problems are bad links (in technical terms: URIs). For further information, see the background information on URIs.
- Broken Cross References
-
Cross-references (internal links) can be broken, e.g. due to missing or misspelled link-targets. See BrokenCrossReferencesChecker
- Missing image files
-
Referenced image files can be missing or misspelled. See MissingImageFilesChecker.
- Missing local resources
-
Referenced local resources (other than images) can be missing or misspelled. See MissingLocalResourcesChecker
- Duplicate link targets
-
link-targets can occur several times with the same name - so the browser cannot know which is the desired target. See DuplicateIdChecker.
- Illegal links
-
The links (aka anchors or URIs) can contain illegal characters or violate HTML link syntax. See IllegalLinkChecker.
- Broken external links
-
External http links can be broken due to myriads of reasons: misspelled, link-target currently offline, illegal link syntax. See BrokenHttpLinksChecker.
- Missing Alt Attribute in Image Tags
-
Images missing an alt-attribute. See MissingImgAltAttributeChecker.
Checking and reporting these errors and flaws is the central business requirement of HSC.
Important terms (domain terms) of html sanity checking is documented in a (small) domain model.