add alternates feature
how about adding alternates feature such as Stylistics Alternates and Contextual alternates?
If a font contains Contextual Alternates, your typesetting program must also allow you to choose to use the alternates. This differs from program to program.
Also, fonts created with Glyphr Studio cannot contain alternates, it isn't supported at this time.
but how tell the font to use this alternatives
This is a great idea. Unfortunately, at the moment we are restricted by the features supported in OpenType.js, and this is one of the OpenType features that is not currently supported by OpenType.js.
It is possible to include alternates in the Unicode "Private Use Area" which is the range 0xE000 to 0xF8FF - they will be part of the font, but they won't behave like OpenType contextual alternates.