spurious space in color feature?

[u-fischer]

Description

The font feature list contains a space and the log for color feature contains a line feed if opacity is not give explictly. While this is valid syntax---and so not a bug---, the log looks odd and the space confuses microtype. So the question is if it is really needed.

Check/indicate

• Relevant for XeTeX
• Relevant for LuaTeX
• Issue tracker has been searched for similar issues?
• Links to <tex.stackexchange.com> discussion if appropriate
  https://tex.stackexchange.com/q/567698/2388

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{fontspec}
\setmainfont[Color=FF0000]{TeX Gyre Heros}

\begin{document}
blub
\end{document}

Further details

The default value of opacitiy is defined in font spec with a space:

  \tl_const:Nn \c__fontspec_opacity_tl {FF~}

This means that if the opacity is not given explictly, there is a space in the feature list, which shows up in the log as line feed:

(fontspec)             <->"TeXGyreHeros:mode=node;script=latn;language=dflt;+tli
g;color=FF0000FF
(fontspec)             ;"