Steps to reproduce the issue

Joomla 4.0-dev @ 16f284f

Login to admin
System -> Installer - Extensions
Wait "forever" for the "Install from web" iframe to load zzzzz
Click Install from Folder tab
Click Check and Install (without having anything in tmp to install)
Note the alert at the top of the page: Warning Capital W

Then go to Install from URL tab
leave the URL field blank
Click Check and Install
Note the alert at the top of the page: warning LowerCase W

Expected result

Consistency, possibly Warning

Actual result

The Javascript rendered alert is warning whereas a PHP rendered alert is Warning

System information (as much as possible)

Safari on Mac

Additional comments