Release 1.03.65.html5-css.251

Development release of the HTML 5 branch, which allows layout using different styles using CSS.

• New html5 transformation uses different CSS styles to control rendering.
• CSS styles can be controlled by adding different variants on top of the main style - RISC OS 2 style, or with different fonts, etc.

Changes since 1.03.65.html5-css.222:

• Definitions including a reason attribute, now include this in the heading in both HTML and HTML 5 transformations.
• HTML 5 variant modern added, to give a more modern look to the manuals.
• HTML 5 variant packed-definitions added, to make the definition entries less sparse.
• CSS for body and headings no longer defaults to monospace.
• New body and heading fonts for Noto Sans, and new headings fonts Saira and Oswald.
• Improved handling of headings and tables to avoid splitting in PDFs.

Fixes since 1.03.65.html5-css.222:

• definition-table headings are generated consistently.
• Error reports from the tool now include a reason field in the backtrace, if one was present.
• docgroup implementation is now consistently used from the chapter, not the root element.
• Indexes now handle the offset and offset-base better.
• Indexes now sort reason code numbers by their value, rather than alphabetically.
• Index headings repeat on continuation pages.
• Tool correctly reports its name and the location of documentation.