[mediaqueries] reevaluating media queries when the environment changes

MQ4 states:

"User agents should re-evaluate media queries in response to changes in the user environment"

MQ3 has the same, phrased differently.

I believe this was intentional, but I cannot think of any good reason
for using should rather than must in this sentence, and would like to
change.

The only two reasons I can think of are:
1) If we write MUST, this will delay MQ3 getting to REC

=> If that's the case, this is no longer relevant, and we can use must

2) We want to allow some user agents to layout and render a static view
of the page once, and then never touch it again (no relayout, no
interaction, no javascript after the initial load...).

=> If that's what we want to do, we can carve out a specific exemption
for such UAs, without allowing normal interactive browsers to fail to
update their media queries when something changes.

Or was there something else?

 - Florian