"indentation is always done by two blank spaces; tabs are not used. This is because tabs have a different space value according to the text editor you use"
Not only is this convention useless (wanna have 2 spaces for a tab? Just set "tabs=2 spaces" in your favourite editor. Wanna have 4? You're free, it won't disturb other developers) but it's even dangerous: spaces are impossible to maintain with a plain editor, while tabs are flexible.
Tabs are democratic: one can change his own display style without interacting with others. Spaces are dictatorial: everyone MUST accept Fabien's style.
The rule "use 2 spaces" has the same meaning of forcing people to use Arial 14pt for their editor just because Fabien does it. Why?
Using tabs would allow Fabien to display his code with 2 spaces tabs, other developers to display with 4 and so on: without drawbacks.
The problem "code with mixed tab and blank indentation is impossible to read." is very simple: don't use spaces. Spaces are fixed, they are the origin of this problem. Use tabs, like every developer is used to do.
You should use tabs exactly because the manual says "tabs have a different space value according to the text editor you use": that is, no matter how you set your tabs (2 spaces, 8 spaces, what you like) the code will always be indented.
And Fabien will always be able to display it with 2 spaces.
Tabs setting (2 spaces, 8 spaces, whatever you want) only changes the way you display the code in your own editor (that is yours) and never alters the code other developers will display.
Spaces alter everyone's display. No choose.
Let developers choose how display their code always respecting a strict indenting rule. Pleace, think about it: there's only one way to gain this flexibility: using tabs.