Allow progressive enhancement using @supports

Author: marek-saji

lib/DoIUse.js

@@ -1,5 +1,6 @@
 import multimatch from 'multimatch';
 
+import postcss from 'postcss';
 import BrowserSelection from './BrowserSelection.js';
 import Detector from './Detector.js';
 
@@ -101,6 +102,38 @@ export default class DoIUse {
         if (overwritees.length > 0) {
           return;
         }
+
+        // find the closest @supports parent
+        /** @type {any} TODO Type this */
+        let supports = usage.parent;
+        while (supports && supports.type !== 'atrule' && supports.name !== 'supports') {
+          supports = supports.parent;
+        }
+        if (supports) {
+          // FIXME This is a very, VERY dirty hack
+          // FIXME Handle e.g. `@supporst (content: ') and ('`)
+          // TODO Report an warning if condition is too complex for us to understand
+          // TODO Support `or`?
+          // TODO Support nested brackets?
+          const temporaryDeclarations = supports.params
+            .split(/(?<=\))\s*and\s*(?=\()/g)
+            .map((part) => part.trim().replaceAll(/^\(|\)$/g, ''))
+            .filter((part) => !/^\s*not\s+/i.test(part));
+
+          const temporaryCss = `i { ${temporaryDeclarations.join(';\n')} }`;
+          const temporaryCssTree = postcss.parse(temporaryCss);
+          const supportsDetector = new Detector([feature]);
+          const safeToUseFeatures = [];
+          supportsDetector.process(
+            temporaryCssTree,
+            ({ feature: supportsFeature }) => {
+              safeToUseFeatures.push(supportsFeature);
+            },
+          );
+          if (safeToUseFeatures.includes(feature)) {
+            return;
+          }
+        }
       }
 
       const messages = [];

test/postcss-progressive-enhancement.js

@@ -20,3 +20,51 @@ test('Progressive enhancement using repeated CSS properties', (t) => {
 
   t.end();
 });
+
+test('Progressive enhancement using @supports', (t) => {
+  const css = `
+    p {
+      display: block;
+    }
+
+    @supports (display: grid) {
+      p {
+        display: grid;
+        flex-wrap: wrap;
+      }
+    }
+  `;
+
+  const result = postcss(new DoIUse({
+    browsers: ['safari >= 9'],
+  })).process(css);
+  const warnings = result.warnings();
+
+  t.equal(warnings.length, 0, 'No warnings');
+
+  t.end();
+});
+
+test('Progressive enhancement using @supports with multiple conditions', (t) => {
+  const css = `
+    p {
+      display: block;
+    }
+
+    @supports (display: flex) and (color: hsl(0, 0%, 0%)) {
+      p {
+        display: flex;
+        background-color: hsl(0, 0%, 0%);
+      }
+    }
+  `;
+
+  const result = postcss(new DoIUse({
+    browsers: ['safari >= 9'],
+  })).process(css);
+  const warnings = result.warnings();
+
+  t.equal(warnings.length, 0, 'No warnings');
+
+  t.end();
+});