Document W3C compliance mode configuration (#85)

Added instructions for configuring W3C compliance mode in tests.

This is a follow up on #84.

Author: mxr576

README.md

@@ -45,6 +45,30 @@ After installation, make sure to commit the `.ddev` directory to version control
     - Ensure you have a working site that has the `weitzman/drupal-test-traits` Composer package.
     - `ddev exec -d /var/www/html/web "../vendor/bin/phpunit --bootstrap=../vendor/weitzman/drupal-test-traits/src/bootstrap-fast.php --printer '\Drupal\Tests\Listeners\HtmlOutputPrinter' ../vendor/weitzman/drupal-test-traits/tests/ExampleSelenium2DriverTest.php"`
 
+### Configuring W3C compliance mode
+
+By default, WebDriver W3C compliance is enabled (`DRUPAL_TEST_WEBDRIVER_W3C=true`). This controls the `w3c` flag in both `MINK_DRIVER_ARGS_WEBDRIVER` and `DTT_MINK_DRIVER_ARGS`.
+
+If you are running tests on Drupal < 11, you may need to disable W3C mode to avoid WebDriver curl errors (HTTP 400) during test runs. You have a few ways to override it:
+
+**Using `ddev dotenv`** (recommended, keeps it out of version control):
+
+```bash
+ddev dotenv set .ddev/.env --drupal-test-webdriver-w3c false
+ddev restart
+```
+
+**Using `.ddev/config.yaml`** (commit this if the whole team needs the same setting):
+
+```yaml
+web_environment:
+  - DRUPAL_TEST_WEBDRIVER_W3C=false
+```
+
+Then run `ddev restart`.
+
+> **Note:** Only disable W3C mode if your tests are failing with HTTP 400 curl errors from the WebDriver endpoint. See [issue #76](https://github.com/ddev/ddev-selenium-standalone-chrome/issues/76) for the original bug report.
+
 ## Watching the tests
 
 ### The easy way: Use noVNC (built-in)