I'm setting up some interesting moments as well as program status changes based on viewing web pages on our website.
When I add the campaign and trigger to say webpage IS and put the url I get the red squiggle underline saying the entry isn't valid. If I put contains then it seems to work however it doesn't appear when we test that the trigger works.
If this is an external page, you're probably prepending the URL with 'http://'. Visits Web Page require that the protocol be removed fom the beginning of the URL so instead of http://www.example.com I would put www.example.com into the trigger. If this is a Marketo LP, though you'll need to use the name of the landing page in your instance instead of the URL.