Aptus is a very useful web design utility for Mac. It can be used to preview what a website will look like in a variety of devices, including iPhones, iPads, and even on specific web browsers on your Mac. The app uses breakpoints, which are a means to analyzing a website in this case. You can add breakpoints by clicking on the wheel at the bottom left corner. You can add as many as you want, and you can choose what browser you want to use for each breakpoint. For example, if you wanted to know what your website looks like on an iPad, you can add a breakpoint that uses the iPad Safari browser and then specify the size of the screen. I noticed that the app doesn't come with preset sizes for the devices that it supports, which could be a nice addition in future versions. Then all you need to do is input the URL of your website to load it. Unfortunately, you can't load local HTML files. You need to have a web server installed on your Mac to do that.
Aptus lets you inspect the code of your website from within the application, which is a feature webmasters will appreciate.
In a nutshell, Aptus is certainly a very useful tool for webmasters who need to know that their websites look great on a variety of devices. There is still some room for improvement, though.