>
recommend having your forearm angled between 90-120°That's what I said ("your forearm should be generally be about horizontal (or a little bit downwards is OK too)"). Never upward.
> which if it's > 90°, an front angulation doesn't make sense at all.
If you want a 10° downwards angle on the keyboard relative to your forearm and your forearm is 5° upwards, then your keyboard still needs 5° downward relative to your desk. If your desk is so high that your forearms are angled up 20° from the horizontal, then something is extremely wrong with your chair/desk height.
And your first link supports exactly what I wrote:
> The natural position of your hands with respect to the relative vertical position at the wrist is along the plane or below it (i.e. you want the tips of your fingers to be at the same height as your wrist or preferably slightly lower). When your hand rises above this plane (making the hand signal for 'STOP'), this is called dorsiflexion or wrist extension. This greatly reduces blood flow through the wrist and can quickly cause pain, fatigue and numbness. Most keyboards have a 'foot' located at the back of the keyboard which is not desirable as it creates a positively inclined keyboarding surface. Many articulating arms offer the option of a negative inclination, which will make the entire work surface slope away from you, ensuring that your hands are not 'bent' upward at the wrist.
And I don't know why you've added another link about sideways mouse slant. That has nothing to do with up-down keyboard/arm angle. If there's some relevant sentence in it, please quote it, because I can find nothing relevant in the publicly available text.
You ask me to "please take your time to read" -- I suggest you take the time to re-read your own first link. It makes clear there is no situation where a keyboard angled upwards (the back of the keyboard elevated) is good ergonomics -- exactly what I said.