There is a hack that will accomplish this, but I seriously advise against using it - your design should adapt to whether someone is in portrait or landscape mode.
Step 1: Query the window.orientation property to see if you're in landscape or portrait mode
Step 2: If you're in portrait mode use a -webkit-transform rotate (-90) on a div that's wrapping your entire page to force it into a landscape layout.
This won't work quite correctly - the browser UI will still be in portrait mode, but presumably the user will figure out that they're supposed to rotate the phone back into landscape mode in order to view the content. This is incredibly annoying to users - there is probably a good reason they are trying to read your site in portrait mode.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…