feat(test-runner): improve error message when no browsers are configured

.changeset/khaki-carpets-tan.md

@@ -0,0 +1,6 @@
+---
+'@web/test-runner': patch
+'@web/test-runner-core': patch
+---
+
+improve error message when no browsers are configured

packages/test-runner-core/src/runner/TestRunner.ts

@@ -39,16 +39,24 @@ export class TestRunner extends EventEmitter<EventMap> {
 
   constructor(config: TestRunnerCoreConfig, groupConfigs: TestRunnerGroupConfig[] = []) {
     super();
+    if (!config.manual && (!config.browsers || config.browsers.length === 0)) {
+      throw new Error('No browsers are configured to run tests');
+    }
+
+    if (config.manual && config.watch) {
+      throw new Error('Cannot combine the manual and watch options.');
+    }
+
+    if (config.open && !config.manual) {
+      throw new Error('The open option requires the manual option to be set.');
+    }
+
     const { sessionGroups, testFiles, testSessions, browsers } = createTestSessions(
       config,
       groupConfigs,
     );
     this.config = config;
 
-    if (this.config.manual && this.config.watch) {
-      throw new Error('Cannot combine the manual and watch options.');
-    }
-
     this.testFiles = testFiles;
     this.browsers = browsers;
     this.browserNames = Array.from(new Set(this.browsers.map(b => b.name)));

packages/test-runner/web-test-runner.config.mjs

@@ -2,4 +2,5 @@ export default {
   rootDir: '../../',
   preserveSymlinks: true,
   nodeResolve: true,
+  browsers: [],
 };