SHOULD appears 12 times and should appears 67 times.

Some of the lower case uses of "should" appear to be in non-normative text that should be notes. But in other cases, it appears to be the equivalent of "SHOULD."

From 3.1 Device Initialization:

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the driver SHOULD set the FAILED status bit to indicate that it has given up on the device (it can reset the device later to restart if desired).
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From 3.2.2 Receiving Used Buffers from the Device

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For each ring, the driver should then disable interrupts by writing VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required.
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The "shoulds" in those two cases have very different meaning for conforming to VIRTIO.

The first is RFC2119 "SHOULD" and the second is just ordinary should, really non-normative.