\definefontfamily
  [mainface]
  [rm]
  [WorkSans]
  [tf={style:light},it={style:lightitalic}]

\setupbodyfont[mainface]

\setupbodyfontenvironment
  [default]
  [a=1.2,
   b=2.4,
   c=3.6,
   d=4.8,
   em=italic]

\starttext
  \startTEXpage[offset=1em]
    roman\\
    {\bf bold}\\
    {\it italic}\\
    {\bi bold italic}\\
    {\tfa roman}\\
    {\bfb bold}\\
    {\itc italic}\\
    {\bid bold italic}\\
  \stopTEXpage
\stoptext