[css-device-adapt] Clarification about device-height and device-width

[karlcow]

The specification doesn't make it clear if width and height can be mixed up.
https://drafts.csswg.org/css-device-adapt/#width-and-height-properties

<meta name="viewport" content="width=device-height">

<meta name="viewport" content="height=device-width">

if they are invalid, they should be treated as step 4.

Other keywords and unknown values translate to 1px

but if they are valid, what do they mean?