fontmin reduce

a.js

var fs = require("fs");
var Fontmin = require('fontmin');

var content = fs.readFileSync('a.txt', 'utf8');

var fontmin = new Fontmin()
    .src('a.ttf')
    .use(Fontmin.glyph({
        text: content,
        hinting: false         // keep ttf hint info (fpgm, prep, cvt). default = true
    }))
    .dest('b');

fontmin.run(function (err, files) {
    if (err) {
        throw err;
    }

    console.log(files[0]);
    // => { contents: <Buffer 00 01 00 ...> }
});

a.sh

mkdir b

cat *.xhtml | grep -oE '\S' | sort | uniq | awk 1 ORS='' > a.txt

ls a.txt
ls a.ttf
ls b

node a.js

ls b