Inspect Element of macOS system App
I bet you know serveral native Apps on macOS is written in HTML.
Use these commands:
defaults write NSGlobalDomain WebKitDeveloperExtras -bool true defaults write -g WebKitDeveloperExtras -bool YES
Open your terminal and execute the two shell commands above.
Check element of macOS "native" Apps
Right click on a "native" App and select "Inspect Element".