When designing applets it is important to allow (as much as possible) for all students to access them.
Fontsize, Color & Contrast
Using large fonts with a good contrast is important. Select a large font-size in Options -> Fontsize before you start designing your applet. You can also use the keyboard shortcut Ctrl + 2 to make all fonts bigger and all lines thicker. Ctrl + 3 will change the default for all new objects to be Black and unfilled.
If you use colors to distinguish otherwise-similar objects, then consider also using dashed lines for one of them. You can also find websites where you can check whether your color scheme is good, for example http://www.vischeck.com/
Make sliders as long as possible so that they are usable by students whose fine motor-skills are less good. Consider adding "decrement" and "increment" Buttons at each end of the slider as well.
Students can use Tab to cycle round the objects in a worksheet you have designed. It is important that you uncheck "Selection Allowed" in Object Properties -> Advanced for the objects you don't want to be selected. There is a keyboard shortcut Ctrl + Shift + D in the desktop version that will toggle this property for objects other than Buttons / Sliders / Checkboxes / Points / Input Boxes.
The arrow keys Left, Right, Up, Down can be used to move Sliders and Points once they are selected.
Space can be used to activate a Button or toggle a Checkbox.
Escape can be used to leave a worksheet and Enter to go back in (useful if you have several GeoGebra worksheets on one page)
GeoGebra Applets are also fully touch-enabled so students can use applets on a special large tablet if necessary.
You can provide custom tooltips for certain objects by using the Caption property, and changing how the tooltip is shown: https://wiki.geogebra.org/en/Tooltips
You can have some text attached to the Graphics Views so that it is read by a screen reader.
If you make a text object in GeoGebra called
altText3D then it will be attached to Graphics View 1, Graphics View 2, Graphics View 3D respectively.