In order to add EMF support in Orca (plotly/orca#152), we had to remove all semi-transparency because EMF does not support it. At the time, we did this work in Orca itself. However, as @etpinard pointed out in plotly/orca#152 (comment)

We could even move this compatibility logic to plotly.js' Snapshot.toSVG - which already does a few similar things. I'm not sure if we should be adding a Plotly.toImage flag (e.g. noOpacity) or add a "format" (e.g. svg-for-emf)?

Thinking about this again, I think adding a noOpacity flag in plotly.js makes sense. There are probably other image formats out there that don't fully support an alpha channel. Now this doesn't solve all the EMF/Inkscape issues; perhaps these remaining issues could be handling in Orca? What do you think @antoinerg ?

@etpinard was clairvoyant in this regard: EPS also does not support semi-transparency. When it's present, it leads to ugly rasterization (see plotly/orca#248 for details).

Therefore I think it would make sense to add a new format to Plotly.toImage to remove all semi-transparent elements. Its name could be svg-opaque.