Hi Sylvain,

this is the same as:

  \definefontfamily[face1][rm][gabriola]

Just in case, "slanted" is an already defined feature. It slants the font.

>     \definefontfamily[face2][rm][gabriola][it={features:{default,
> effect={}}}]
>     \definefontfamily[face3][rm][gabriola][it={features:{default,
> effect={width=1.0}}}]

BTW, the documentation is clear
(http://texdoc.net/texmf-dist/doc/context/presentations/bachotex/2018/bachotex-2018-fonteffects.pdf#search=fontfeature):
font features should be defined as such before being deployed.

> Could someone explain:

I will try from what I understand.

> 1. What does "effect" mean?

A ConTeXt (and not OpenType) font feature?

> Why does "effect={}" embolden the fonts even though nothing ({}) is
> given as arguments?
"effect={}" may be reading default values (it might be intended behavior
or not).

But it also could be reading or calculating from previously defined values.

> 2. Similarly, what does "width" do? Why does "width=1.0" shrink the
> width of the line instead of doing nothing?

This might be a bug, because of all the code improvements.

I’m reporting a proper bug.