\starttext

    \definefontfeature[redboxed]  [default][boundingbox=palered]
    \definefontfeature[greenboxed][default][boundingbox=palegreen]
    \definefontfeature[blueboxed] [default][boundingbox=paleblue]

    \definefont[TestFontA][Serif*redboxed]
    \definefont[TestFontB][Serif*greenboxed]
    \definefont[TestFontC][Serif*blueboxed]

    \startTEXpage[offset=10pt]

        \start \TestFontA \input zapf \stop \blank
        \start \TestFontB \input zapf \stop \blank
        \start \TestFontC \input zapf \stop

    \stopTEXpage

\stoptext
