What is left to do

Add an [imgurl] tag?
Manage lightscript (or even markdown?) as output languages;
Check where the errors are to display them to the user:
- Count character offset, line number and column number in the lexer;
- Produce readable exceptions;
- Make a clean interface to transmit them;
Check why exceptions on raw tags effectively escape the content, as it shouldn't…?
Look for security flaws (we really don't want stored XSS flaws!).
Implement match names (such as \[\*+\] for lists).
Manage keywords with tags such as [tag key=value other="something else"].