[Tex/LaTex] Microtype and word spacing

Question

I'm trying to change the default word spacing for some font. I know 2 ways to do it: setting \fontdimens or \spaceskip. With these approaches and microtype's \textls I get two different results, none of them satisfactory. How can I get the same (increased) interword space inside letterspaced text and between it and regular text?

Example:

\documentclass{book}

\def\test#1{{#1\relax
\textls[200]{test test} test test\par}}

\usepackage{microtype}

\begin{document}
\test{}
\test{\spaceskip 10pt}
\test{\spaceskip 0pt\fontdimen2\font 10pt}

\end{document}

Answer 1

The question has multiple non specified assumptions. But in case you want to increase the default interword space, use XeTeX. See section 7.3 of fontspec's manual.

\documentclass[12pt]{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\usepackage{kantlipsum}
\usepackage{microtype}
\begin{document}
\kant[1]
\addfontfeature{WordSpace=10}
\kant[1]
\textit{\kant[1]}
\end{document}