The frames versions of the HTML manuals have some accessibility errors that are quite easy to fix, involving the index.html file and section heads.
The index.html file for the frames version has several errors:
1) The <html> element lacks a lang attribute (lang="en" in this case).
2) The <frameset> element lacks a <noframes> element telling people with browsers that don't support frames where to find an alternative version of the documentation. We can point them to the single-file HTML manual.
3) The <frameset> and <frame> elements require title attributes.
It is possible to fix these issues fairly easily by providing a modified version of the DITA-OT126.96.36.199/resource/index.html file and then by making some additional edits to the init and html.dita tasks in the build.xml file.
The documentation also violates the rule that heads must appear in order. The topics in the Derby documentation generate only two HTML head tags, <h1> for the topic title and <h4> for sections. Sections cannot be nested in DITA.
It is possible to fix this problem by providing a modified version of the DITA-OT188.8.131.52/xsl/xslhtml/dita2htmlImpl.xsl file that specifies the use of <h2> tags instead of <h4>. Section heads are always generated with the "sectiontitle" class, which is specified in the .css files, so the output seems to look the same with either head in a browser that supports stylesheets.
I plan to file a patch for this task tomorrow.