I love to listen to YouTube videos like Podcasts while working. I bet many people do have this habit.
to clickClassName(theClassName, elementnum)
tell application "Google Chrome" to (tabs of window 1 whose URL contains "youtube")
set youtubeTabs to item 1 of the result
tell application "Google Chrome"
execute youtubeTabs javascript "document.getElementsByClassName('" & theClassName & "')[" & elementnum & "].click();"
end tell
end clickClassName
clickClassName("ytp-play-button ytp-button", 0)
The above AppleScript is for Google Chrome.