@ClaudioFiandrino ... *still holding breath, can't last much longer * ...

@AndrewStacey Sorry, yesterday and this morning I worked a bit on my switching architectures library github.com/cfiandra/Sa-TikZ (very proud of having understood is choice for a simple boolean usage - setting or not nodes in math mode automatically- .. yes: I'm doing very short steps each time :D ). I'm going to have a deeper look on the novelties of the library :)

@AndrewStacey: Now the usage it's really more intuitive even if I still don't understand a couple of things: for the highlighter part right, left, top, bottom margin seem to not work properly and for the fader part why changing cap nothing happens.

Oh, I've also introduced the command \tikzmarkset{fader/highlighter}{options}: it seems to me a good way to set globally some options and then just redefine locally others (for example set globally the color then just modify locally dimensions).

The code I played with:

\documentclass[a4paper]{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\usetikzmarklibrary{highlight}

\newcommand{\tikzmarkset}[2]{
	\pgfkeys{/tikz/#1/.cd,#2}
}

\begin{document}
\[\highlight[colour=green!50]{x+y}=100\]

\tikzmarkset{highlighter}{left margin=1ex,right margin=1ex, top margin=3ex, bottom margin=4ex,colour=cyan!50}
\[\highlight{x+y}=100\] % same as before? Strange!

\tikzmarkset{highlighter}{colour=orange!50,initial offset=1pt,final offset=1pt}
\[\highlight[final depth=7pt]{x+y}=100\]

@ClaudioFiandrino The left, right, top, bottom only come into effect if the highlighting reaches the margin. Then they specify how much the highlighting goes into the appropriate margin. So with highlighting that doesn't reach the margin they have no effect. To increase the left/right on an inline highlight then you need to use the initial offset and final offset. The cap option is no longer implemented as the method is completely different to before.

The highlighter now works by drawing a region and filling it, whereas before it worked by drawing lots of lines.