[css-grid] computed grid-row-start with default span integer

[ewilligers]

https://drafts.csswg.org/css-grid/#grid-placement-span-int

span && [ <integer> || <custom-ident> ]
If the <integer> is omitted, it defaults to 1. Negative integers or zero are invalid.

grid-row-start "span i" computes to "span 1 i" on Blink/WebKit, but computes to "span i" on Edge/Firefox.

grid-row "span i / auto" computes to "span 1 i / auto" on Blink/WebKit, but computes to "span i / auto" on Edge, empty string on Firefox.

For many properties, the shortest serialization principle often leads to keywords being omitted, but it is unusual for numeric values to be omitted.

[tabatkins]

None of these are "computing to" anything. They're serializing as "span i" or "span 1 i". All browsers compute them to the same internal value.

Per the shortest serialization principle, the 1 should be omitted from the serialization here. There's nothing special about numeric values vs keywords; numerics get omitted all the time (in shorthands, particularly).

[ewilligers]

http://jsfiddle.net/ericwilligers/nawfLdp8/

When we read the specified value, all browsers give "span i" and "span i / auto"

When we call getComputedStyle(), Blink and WebKit give "span 1 i" and "span 1 i / auto", while Edge and Firefox omit the 1.

[mrego]

JFTR, Chromium bug related to this issue: http://crbug.com/916827