A textbox is a great feature, especially for computer users who will struggle to write with just a mouse, but there are a few issues with it:

1. The marker style is probably supposed to resemble actual writing, but it really doesn't work well in a math context. For example, it may be difficult to line up addition and subtraction equations. I would recommend a monospaced font like Inconsolata instead, if not an option to choose a font.
2. Because the letters are forced to be capitalized, some letters have issues. For example, 1 and i are hard to distinguish, and so are 0 and O.
3. The color feature will only word in separate textboxes.
4. More features like subscripts and superscripts (I believe they are decently easy to make in HTML) would be appreciated.
5. More of a suggestion, but maybe some type of TeX support for symbols not on a standard keyboard?