index.js

'use strict';
var lodash = require('lodash');
var parseAttrs = require('posthtml-attrs-parser');

module.exports = function (options) {

    var pxRegex = /(\d*\.?\d+)px/ig;

    options = lodash.extend({
        rootValue: 16, // root font-size
        unitPrecision: 5, // numbers after `.`
        minPixelValue: 0 // set it 2 if you want to ignore value like 1px & -1px
    }, options);

    function createPxReplace(rootValue, unitPrecision, minPixelValue) {

        return function (m, $1) {

            // ignoring `PX` `Px`
            if (m.indexOf('px') === -1) {
                return m;
            }
            if (!$1) {
                return m;
            }
            var pixels = parseFloat($1);
            if (pixels < minPixelValue) {
                return m;
            }
            return toFixed((pixels / rootValue), unitPrecision) + 'rem';
        };
    }

    function toFixed(number, precision) {
        var multiplier = Math.pow(10, precision + 1),
            wholeNumber = Math.floor(number * multiplier);
        return Math.round(wholeNumber / 10) * 10 / multiplier;
    }

    return function px2rem(tree) {

        var pxReplace = createPxReplace(options.rootValue, options.unitPrecision, options.minPixelValue);

        // inline CSS `<style>` in HTML
        tree.match({tag: 'style'}, function (node) {

            var replacedContent;
            replacedContent = node.content.toString().replace(pxRegex, pxReplace);
            node.content = replacedContent;

            return node;

        });

        /* inline CSS `style="xxx"` in DOM*/
        tree.match({attrs: {style: true}}, function (node) {

            var attrs = parseAttrs(node.attrs);
            for (var x in attrs['style']) {

                if (attrs['style'].hasOwnProperty(x)) {
                    var styleValue = attrs['style'][x];

                    // e.g. style="width=10px; width=20px;"
                    if (typeof styleValue == 'object')
                        styleValue = styleValue[styleValue.length - 1];

                    var newStyleValue;
                    newStyleValue = styleValue.toString().replace(pxRegex, pxReplace);

                    attrs['style'][x] = newStyleValue;
                }

            }

            node.attrs = attrs.compose();

            return node;
        });

        return tree;
    };
};