Probably the one part of the phone that most of us don't pay attention to is the Accessibility options. However they do have some great features in there that help out everyone. HTC has mostly kept it the same from AOSP Android, but there are a few extra features wrapped inside. HTC has kept the TalkBack feature, which I think every phone has. That feature allows your phone to read everything off to you. A great feature for those that might be blind.
That's not all that HTC has put in the HTC One M9 though. There's plenty more for accessibility. Including the Font size. So now you can increase the font size within their apps. Unfortunately, you can't set it system wide. You can also put captions on images. There's also magnification gestures. Which allow you to zoom in on screen by triple tapping. And from there you can pan around or pinch to adjust the zoom level. HTC also has Color inversion. So you can make whites dark and blacks light. Obviously, that may impact the performance of the HTC One M9. Another one to do with color is color correction. It aids those who have various levels of color blindness. There's also Notification reminder which will ping you repeatedly if you have pending notifications. Here you can set that from 15 seconds to as long as 5 minutes. You can also use the power button to end the call.
These are just some of the very, very many accessibility options available in the HTC One M9. And they aren't just for disabled users, as there are quite a few features in there that I have turned on myself, just because it's a better experience for myself. Which is what HTC is going for here. Having said that, there's a lot of really cool features here for those that are deaf, blind or even color blind. Which is a nice job on HTC's part.