\setuplayout[grid=yes,marking=on]
\showgrid
\showbaselines

\starttext
Hello world!

\placeongrid[bottom,line]{
  \framedtext[
    width=\textwidth,
    background=screen,
    backgroundcorner=round,
    framecorner=round,
    frame=off]{
         Hello world!
    }
}

Hello world!

\startlinecorrection
\framed{Hello world!}
\stoplinecorrection

Hello world!
\stoptext