I have added a missing copyright statement.
Thanks.
toml.hpp is generated. Please regenerate it from source and possibly exclude it.
toml.hpp isn't shipped in the package (the Meson build system installs the "regular" flavour, not the "single header" once; see the README for details). Is this necessary?
In debian/rules you reference files from /usr/share/javascript/highlight.js but there is no Depends on libjs-highlight.js. Why do you have that README.html in the first place?
Good catch, I'll add it to the Recommends. As for why README.md is installed as README.html, Policy 12.4[1] says that "If the package comes with extensive documentation in a markup format that can be converted to various other formats you should if possible ship HTML versions in a binary package". As Markdown was meant to be used to be converted to HTML since its creation this seems to me like a natural thing to do.
While adding highlight.js to the Recommends, I've also added automatic dark and light theme handling using the CSS prefers-color-scheme media query, so that my eyes won't melt again while reading the installed documentation :)