Way to maintain same screen ratio on data projector as on Mac laptop
Your projector probably has a maximum native resolution of XGA or 1024x768 (standard iPads have a resolution of 1024x768 points). Some projectors will allow you to send a higher resolution signal and scale it down for you. Most projectors I have seen don't do a great job, but it at least functions. To see if your projector will do that:


Connect your Mac to the projector
Ppen the Displays panel in System Preferences
Option-click the "Scaled" option for the resolution of the projector. 
Select a 4:3 resolution higher than 1024-768 (e.g. 1280x1024)


If the projector goes black, wait and your Mac will timeout back to 1024x768. 

The other option is to purchase a VGA scaler (For example, the Kramer VP-426 takes VGA input and then outputs VGA). You need hardware that will take  one of the various formats your Mac can output and output at XGA (1024x768). What the hardware will do is let your Mac output a higher resolution, forcing the web page to render the "desktop" size, but output it to the smaller resolution projector. But honestly, purchasing a scaler for this purpose is overkill.