build_factor.mk4

-- https://github.com/michal-h21/tex4ebook/issues/113#issuecomment-1681960392

local domfilter = require "make4ht-domfilter"

local process = domfilter {
  function(dom)
    for _, image in ipairs(dom:query_selector(".equation img")) do
      local src = image:get_attribute("src")
      if src then
        local m = io.popen("identify -ping -format '%w' " .. src, "r")
        local result = m:read("*all")
        m:close()
        print(result)
        -- change this to your liking
        -- local factor = 0.9
        -- local factor = 0.8
        -- local factor = 0.6 -- doesn't shrink x = ax + by + cz noticably.
        -- local factor = 0.5
        local factor = 0.4
        -- local factor = 0.3 -- too small (for \grad F = J in preface)
        local newwidth = (tonumber(result) or 0) * factor
        if newwidth > 0 then
          local style = image:get_attribute(style) or ""
          style = style .. string.format("width:%spx;" , newwidth)
          image:set_attribute("style", style)
        end
      end

    end

    return dom
  end
}

Make:match("html$", process)