The XhtmlValidator is a PHP class that can be used to validate XHTML documents. It uses only the expat extension functions, which are always available in PHP, so it does not need other external XML processing extensions. The class parses the documents and checks whether the tags and attributes used by the documents are allowed within the XHTML standard. If validation errors are found, highlighted error messages and offending document line numbers will be returned.
Tartan is a text parsing engine targeted at wiki text. The syntax specification is defined in YAML in the form of regex-based rules. It supports layering and multiple output types. Rules for Markdown to HTML are included, with optional layered extensions for tables. It is implemented in Ruby, but looking to have implementations in other languages.