The cheap solution: You can enable font features globally by adding them to the default feature set:
%% The “default” set is always active.
\definefontfeature [default] [default] [
liga=yes,
hlig=yes,
dlig=yes,
]
\definetypeface[Libertine][rm][Xserif][Linux Libertine O]
\setupbodyfont[Libertine, 9pt]
\starttext
fi ffl ct st
\stoptext
For more precise control over when the ligatures are activated you will need a typescript.
This lets you specify which feature sets are enabled for each type face individually. But you could as well turn ligatures off for, say, the bold face by requesting features=default in the relevant line. Below example applies the ligature set to all four serif faces:
%% 1. Create a feature set, inheriting the defaults.
\definefontfeature [extraligatures] [default] [liga=yes,script=latn,hlig=yes,dlig=yes]
%% 2. Map font files for Libertine to type faces, applying the “extraligatures” features.
\starttypescript [serif] [mylibertine]
\definefontsynonym[Serif] [file:LinLibertine_R] [features=extraligatures]
\definefontsynonym[SerifItalic] [file:LinLibertine_RI] [features=extraligatures]
\definefontsynonym[SerifBold] [file:LinLibertine_RB] [features=extraligatures]
\definefontsynonym[SerifBoldItalic] [file:LinLibertine_RBI] [features=extraligatures]
\stoptypescript
% 3. Define corresponding type face identifier.
\definetypeface [mylibertine] [rm] [serif] [mylibertine] [default]
% 4. Use the font in the document.
\setupbodyfont[mylibertine]
% 5. Test all four defined styles.
\def\ligatest#1{\bgroup#1fi ffl ct st\egroup\par}
\starttext
\processcommacommand[,\italic,\bold,\bolditalic]\ligatest
\stoptext
NB: Xetex neither has good support in ConTeXt nor is it widely used. You might consider switching to MkIV which comes with greatly improved font handling facilities and is actively developed.
