Behat test fails, unable to find Username field using @javascript
I bet you are either one:

Testing against a non-English site.
Having BigPipe enabled.
Missed to tag that scenario with @api.
Misconfigured Selenium.


To solve 1. you need to put the non-English strings into your behat.yml like so:
Drupal\DrupalExtension:
  text:
    log_out: &quot;Sign out&quot;
    log_in: &quot;Sign in&quot;
    password_field: &quot;Enter your password&quot;
    username_field: &quot;Username&quot;

@see docs Override text strings.

To solve 2. which can occur when BigPipe somehow changes the markup in an undesired way, you can try out the patch at https://github.com/jhedstrom/drupalextension/pull/325.patch.
@see pull request Big pipe compatibility follow-up

To solve 3. simply add an @api tag to your scenario.

To solve 4. you must point wd_host to http://dev.mysite.com:4444/wd/hub, since your base_url is http://dev.mysite.com.
wd_host: &quot;http://dev.mysite.com:4444/wd/hub&quot;