HTML generation requires kramdown, easily installed as a rubygem. Rubygems installation should satisfy dependencies automatically.
HTML generation can then be done from the project root:
$ kramdown doc/webp-container-spec.txt --template doc/template.html > \ doc/output/webp-container-spec.html
kramdown can optionally syntax highlight code blocks, using CodeRay, a dependency of kramdown that rubygems will install automatically. The following will apply inline CSS styling; an external stylesheet is not needed.
$ kramdown doc/webp-lossless-bitstream-spec.txt --template \ doc/template.html --coderay-css style --coderay-line-numbers ' ' \ --coderay-default-lang c > \ doc/output/webp-lossless-bitstream-spec.html
Optimally, use kramdown 0.13.7 or newer if syntax highlighting desired.