The width CSS descriptor is shorthand for setting both the min-width and the max-width descriptors of the viewport. By providing one viewport length value, that value will determine both the min-width and the max-width to the value provided.
If two viewport values are provided the first value will be set to the min-width and the second value will be set max-width.
/* An example with one viewport value: */
@viewport {
    width: 320px;
}
/* An example with two viewport values: */
@viewport {
    width: 320px, 120px;
}
 auto<length><percentage>| Related at-rule | @viewport | 
|---|---|
| Initial value | as each of the properties of the shorthand: | 
| Percentages | as each of the properties of the shorthand: | 
| Computed value | as each of the properties of the shorthand: | 
<viewport-length>{1,2}where
<viewport-length> = auto | <length-percentage>where
<length-percentage> = <length> | <percentage>
@viewport {
  width: 500px;
} | Specification | Status | Comment | 
|---|---|---|
| CSS Device Adaptation The definition of '"min-width" descriptor' in that specification.  |  Working Draft | Initial definition | 
| Desktop | ||||||
|---|---|---|---|---|---|---|
width descriptor | 
29 — 84 |  12
  | 
No |  10
  | 
 16
  | 
No | 
| Mobile | ||||||
|---|---|---|---|---|---|---|
width descriptor | 
4.4 — 37 | 29 — 84 | No |  16
  | 
No | 2.0 | 
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    https://developer.mozilla.org/en-US/docs/Web/CSS/@viewport/width