\setupTABLE inherits from \setupframed which has an orientation option. So \bTR[orientation=90] on the header row should do it, with the content specified normally i.e. without using \RtH. A quick test on one of my tables showed some issues with alignment so you may need to fix the 'width' (now the height) of the cells rather than let it be calculated by contents.
On 15 Mar 2023, at 18:15, Henning Hraban Ramm via ntg-context
wrote: Hi!
In my table, I want the header texts to be 90˚ rotated and aligned to the bottom of their cell. I couldn’t find an option to do that. Tried \dontleavehmode or \hfill in my \RtH macro to no avail. Is it possible?
\define[1]{\RtH}{\rotate{#1}}
\starttext
\setupTABLE[offset=3pt] \setupTABLE[header][background=color,backgroundcolor=yellow,align=bottom] \bTABLE \bTABLEhead \bTR \bTH\RtH{animal}\eTH \bTH\RtH{size}\eTH \bTH\RtH{weight}\eTH \bTH\RtH{average speed}\eTH \eTR \eTABLEhead \bTABLEbody \bTR \bTD sparrow \eTD \bTD small \eTD \bTD light \eTD \bTD high \eTD \eTR \bTR \bTD dog \eTD \bTD medium \eTD \bTD medium \eTD \bTD medium \eTD \eTR \bTR \bTD snail \eTD \bTD small \eTD \bTD light \eTD \bTD slow \eTD \eTR \bTR \bTD elephant \eTD \bTD big \eTD \bTD heavy \eTD \bTD medium \eTD \eTR \eTABLEbody \eTABLE
\stoptext
— Bruce Horrocks Hampshire, UK