`make4ht`: increasing font size

Question

I have a simple *.tex file for this question:

\documentclass[12pt,letterpaper]{article}

\usepackage{lmodern}
\usepackage[T1]{fontenc}

\usepackage{amsmath}

\begin{document}

\title{A test \LaTeX to HTML page}

\section{Section One}
    Hello world. Here is some mathematics:
        \begin{equation}
            x + y = 3
        \end{equation}

\end{document}

Now, when I increase font size in the \documentclass command from 12pt to 16pt, I don't see a corresponding increase in font size in the output HTML.

Why is that?

Here is my build file:

local filter = require "make4ht-filter"
local process = filter{"cleanspan", "fixligatures", "hruletohr"}
if mode == "draft" then
  Make:htlatex()
else
  Make:htlatex()
  Make:htlatex()
  Make:htlatex()
end
Make:image("png$",
"dvipng -bg Transparent -T tight -o ${output}  -pp ${page} ${source}")
Make:match("html$",process)
Make:match("html$", "tidy -m -xml -utf8 -q -i ${filename}")

Answer 1

Your example doesn't work even in pdf mode, standard article class doesn't support 16pt option. You can use scrartcl but it will probably change appearance of your document. Anyway, here is a modified example:

\documentclass[fontsize=16pt, paper=letter]{scrartcl}
\usepackage{lmodern}
\usepackage[T1]{fontenc}

\usepackage{amsmath}

\begin{document}

\title{A test \LaTeX to HTML page}

\section{Section One}
    Hello world. Here is some mathematics:
        \begin{equation}
            x + y = 3
        \end{equation}

\end{document}

this works in pdf mode, but tex4ht uses fixed CSS template by default, so the changed font size isn't used in the html. You can use custom config file and configure the font size through css:

\Preamble{xhtml}
\Css{body{font-size:1.3em;}}
\begin{document}
\EndPreamble

Note that it is better to use em dimensions in css instead of `pt. You can use the config file:

make4ht -c config.cfg filename.tex

the result: