Use less hacky fix for urls detected as custom properties

src/lib/generateRules.js

@@ -262,11 +262,37 @@ function parseRules(rule, cache, options = {}) {
 const IS_VALID_PROPERTY_NAME = /^[a-z_-]/
 
 function isValidPropName(name) {
-  // TODO: properly fix this!
-  return IS_VALID_PROPERTY_NAME.test(name) && !name.startsWith('http')
+  return IS_VALID_PROPERTY_NAME.test(name)
+}
+
+/**
+ * @param {string} declaration
+ * @returns {boolean}
+ */
+function looksLikeUri(declaration) {
+  // Quick bailout for obvious non-urls
+  // This doesn't support schemes that don't use a leading // but that's unlikely to be a problem
+  if (!declaration.includes('://')) {
+    return false
+  }
+
+  try {
+    const url = new URL(declaration)
+    return url.scheme !== '' && url.host !== ''
+  } catch (err) {
+    // Definitely not a valid url
+    return false
+  }
 }
 
 function isParsableCssValue(property, value) {
+  // We don't want to to treat [https://example.com] as a custom property
+  // Even though, according to the CSS grammar, it's a totally valid CSS declaration
+  // So we short-circuit here by checking if the custom property looks like a url
+  if (looksLikeUri(`${property}:${value}`)) {
+    return false
+  }
+
   try {
     postcss.parse(`a{${property}:${value}}`).toResult()
     return true