--- /dev/null
+/* @flow */
+"use strict";
+
+const keywordSets = require("../reference/keywordSets");
+const valueParser = require("postcss-value-parser");
+
+/**
+ * Check if a word is a font-size value.
+ */
+module.exports = function(word /*: string*/) /*: boolean*/ {
+ if (!word) {
+ return false;
+ }
+
+ if (keywordSets.fontSizeKeywords.has(word)) {
+ return true;
+ }
+
+ const numberUnit = valueParser.unit(word);
+ if (!numberUnit) {
+ return false;
+ }
+
+ const unit = numberUnit.unit;
+
+ if (unit === "%") {
+ return true;
+ }
+ if (keywordSets.lengthUnits.has(unit.toLowerCase())) {
+ return true;
+ }
+
+ return false;
+};