I think they're just escaping the hyphen, I don't know of any hack that might use that [there are many "backslash hacks"] other than perhaps for IE5.5 (on Mac?). You're not supposed to start class names with hyphens by convention so there could be a pre-parser/checker that complains if it's not escaped?

I thought classes starting with an hyphen are reserved for browser vendors[1]. Maybe this changed since the last time I checked the spec.

[1] = https://www.w3.org/TR/CSS21/syndata.html#vendor-keywords

Afaik it's just for escaping `-`

It means that this `.-stretch` needs a class `.Grid` on the same element