The client side form validation messages in com_users login view, are actually generated by the browser, not by Joomla!. This is because the submit button is missing the "validate" CSS class. Furthermore, the layout does not load the required JavaScript files.
Though most browsers interpret the required attribute and offer some form of client side validation / visualisation, custom registered validation handlers like "validate-username" assigned to the username field are not recognised.
Testing Instructions
Either go to: http(s)://domain.tld/index.php?option=com_users&view=login or create a menu item to the login form
Hit the "Log in" button. Depending on your browser, you should see a message that indicates an invalid required field (e.g. Username).
Install / Apply the patch and refresh the page. You should now see the standard validation behavior and messages.
Leave Username empty, using Firefox 45.0.1, OS X 10.11.4
Test without Patch: Got "Please fill in this Field"-Info near Username-Field
Test without Patch: Got red Message "Invalid field: Username" above Form.
joomla-cms-bot
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successfully on 4e72df1
Kubik-Rubik
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rdeutz
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I have tested this item
successfully on 4e72df1
Leave Username empty, using Firefox 45.0.1, OS X 10.11.4
Test without Patch: Got "Please fill in this Field"-Info near Username-Field
Test without Patch: Got red Message "Invalid field: Username" above Form.
This comment was created with the J!Tracker Application at issues.joomla.org/joomla-cms/9607.