index.js

var postcss = require('postcss');
var units = require('units-css');
var parseCssFont = require('parse-css-font');
var assign = require('object-assign');

module.exports = postcss.plugin('postcss-increase-text-size', function (opts) {
	opts = assign({
		fontSizeMultiplyBy: 1.2,
		lineheightMultiplyBy: 1.2,
		selectorsBlackList: []
	}, opts);

	function propInArray(array, prop) {
		return array.indexOf(prop) > -1;
	}

	return function (css, result) {
		css.walkDecls(function (decl) {

			if (propInArray(opts.selectorsBlackList, decl.parent.selector)) {
				return;
			}

			if (decl.prop === 'font') {

				var fontValue = parseCssFont(decl.value);
				var fontSize = units.parse(fontValue.size);
				var lineHeight = fontValue.lineHeight;
				var fontValueCon;

				if (fontSize.unit == 'em' || fontSize.unit == 'rem') {
					fontSize = fontSize.value * opts.fontSizeMultiplyBy + fontSize.unit;
				} else {
					fontSize = Math.round(fontSize.value * opts.fontSizeMultiplyBy) + fontSize.unit;
				}


				if (lineHeight != 'normal') {
					var newlineHeight = units.parse(lineHeight);

					if (newlineHeight.unit == 'em' || newlineHeight.unit == 'rem') {
						newlineHeight = newlineHeight.value * opts.lineheightMultiplyBy + newlineHeight.unit;
					} else {
						newlineHeight = Math.round(newlineHeight.value * opts.lineheightMultiplyBy) + newlineHeight.unit;
					}

					fontValueCon = fontSize + '/' + newlineHeight;
				} else {
					fontValueCon = fontSize;
				}

				decl.value = decl.value.replace(/(\d\S*)(\d+)?/g, fontValueCon);
			}

			if (decl.prop === 'font-size') {
				var propUnit = units.parse(decl.value);

				if (propUnit.unit == 'em' || propUnit.unit == 'rem') {
					propUnit.value *= opts.fontSizeMultiplyBy;
				} else {
					propUnit.value = Math.round(propUnit.value * opts.fontSizeMultiplyBy);
				}

				decl.value = propUnit.value + propUnit.unit;
			}

			if (decl.prop === 'line-height') {
				var propUnit = units.parse(decl.value);
				if (propUnit.unit == 'em' || propUnit.unit == 'rem') {
					propUnit.value *= opts.lineheightMultiplyBy;
				} else {
					propUnit.value = Math.round(propUnit.value * opts.lineheightMultiplyBy);
				}
				decl.value = propUnit.value + propUnit.unit;
			}
		});
	};
});