How to render video and text to make a new video

I want to make a app that has some videos and music in library that people and choose and then add some text on it. I want to know how to render that video with the text that the user will type. For now it will be a web app. I am have a html template with some text effects and after the user will provide the text i will add that into that html but how am i going to render that template with the video