You need to play the video on Apple devices (iPhone, iPad, Mac QuickTime).

To maintain "High Definition" quality, ensure your output settings use the or H.265 (HEVC) codec. These are the engines inside the MP4 container that balance file size and clarity. If you are using a converter, look for an option to keep the "Original Resolution" and "Constant Quality." Quick Comparison: WebM vs. MP4 Developer Main Use HTML5 Video/Streaming Universal Playback/Storage Compatibility Browsers, Android Almost every digital device File Size Extremely Small Small to Medium Transparency Supports Alpha Channel Limited Support

I can give you a step-by-step walkthrough for the specific tool that fits your needs.

Technically, every time you "transcode" (change the format) of a compressed video, there is a theoretical loss in data. However, if you use the right settings, the human eye won't notice a difference.