Screem has support for looking at the !DOCTYPE tag and reading the DTD file, if present on the system, or downloading the DTD if not. Other features, such as inline tagging for example will then use the DTD to decide what to present to you.
The following features utilise DTD support
inline tagging
auto closing tag completion
invalid element warning
element properties view
Currently there are problems parsing some DTDs and so only simple ones, such as HTML will probably parse correctly, others such as Docbook are known to have some problems.