Gavin - I suggest adding <meta name="viewport" content="width=device-width,initial-scale=1"> to the standard HTML <head>...</head> block. This will make the result noticeably better on mobile/pads and is, so far as I have seen, harmless on larger screens.
It looks like this would be in tp/Texinfo/Convert/HTML.pm, where the charset http-equiv is output. FWIW ... --best, karl.
