Add SQL query for custom property adoption analysis

Author: aarontgrogg

sql/2025/css/custom_property_adoption.sql

@@ -0,0 +1,59 @@
+#standardSQL
+# % of sites that use custom properties.
+# Same query as 2019, to compare trend
+CREATE TEMPORARY FUNCTION usesCustomProps(css JSON)
+RETURNS BOOLEAN LANGUAGE js AS '''
+try {
+  var reduceValues = (values, rule) => {
+    if ('rules' in rule) {
+      return rule.rules.reduce(reduceValues, values);
+    }
+    if (!('declarations' in rule)) {
+      return values;
+    }
+
+    return values.concat(rule.declarations.filter(d => d.property.startsWith(`--`)));
+  };
+  return css.stylesheet.rules.reduce(reduceValues, []).length > 0;
+} catch (e) {
+  return false;
+}
+''';
+
+SELECT
+  client,
+  COUNTIF(num_stylesheets > 0) AS freq,
+  total,
+  COUNTIF(num_stylesheets > 0) / total AS pct
+FROM (
+  SELECT
+    client,
+    page,
+    COUNTIF(usesCustomProps(css)) AS num_stylesheets
+  FROM
+    `httparchive.crawl.parsed_css`
+  WHERE
+    date = '2025-07-01' AND
+    rank <= 1000000 AND
+    is_root_page -- remove if wanna look at home pages AND inner pages. Old tables only had home pages.
+  GROUP BY
+    client,
+    page
+)
+JOIN (
+  SELECT 
+    client, 
+    COUNT(0) AS total 
+  FROM 
+    `httparchive.crawl.pages` 
+  WHERE
+    date = '2025-07-01' AND
+    rank <= 1000000 AND
+    is_root_page -- remove if wanna look at home pages AND inner pages. Old tables only had home pages.
+  GROUP BY 
+    client
+)
+USING (client)
+GROUP BY
+  client,
+  total