\setupbodyfont[dejavu]

\startluacode

    -- we could populate this one mostly automatic if needed
    -- but also expect a font to have such kerns

    local kern = -50
    local pair = { [32] = kern }

    fonts.handlers.otf.addfeature {
        name    = "kern", -- spacekerns assume kern (for now)
        type    = "kern",
        data    = {
            D = pair,
            E = pair,
            F = pair,
            [32] = {
                D = kern,
                E = kern,
                F = kern,
            },
        }
    }

\stopluacode

\starttext

\enabledirectives[fonts.injections.usespacefontkerns]

\definecolor[tgray][s=.5,t=.5,a=1] \showfontkerns \showfontitalics \showglyphs

\definefontfeature[kern] [mode=node,kern=yes,spacekerns=yes]

\definedfont[file:dejavu-serif.ttf*kern @ 48pt]
\definedfont[file:dejavu-serif.ttf*default @ 48pt]

\startTEXpage[offset=10pt,foregroundcolor=tgray]
    A B C D E F G H
\stopTEXpage

\stoptext