HomeworkMan is specifically designed for mobile devices. It is not even a website and exists only in app form. I have found, in terms of mobile technology, HomeworkMan to be one of the most useful resources I have come across. The app is an assistive tool for students in middle and high school courses and provides easy constant access to formulas in math, physics, and chemistry, in addition to unit conversion and English grammar. It is not assignment or assessment based, but it is a tool designed for students to easily access and carry information that is often found in different places. You can also plug information into the formulas instead of typing it out on calculators allowing for application rather than calculation.
https://apps.apple.com/us/app/homeworkman/id1546412142
You need to download the app to see it, as there is no web browser version.