Add shorthand and longhand property styleguide entry (#1891)

Author: jamesnw

docs/guidelines.md

@@ -195,6 +195,18 @@ Omit "is used" where there's no loss in meaning.
 For example, prefer "The feature reads…" over "The feature is used to read…"
 ([#727](https://github.com/web-platform-dx/web-features/pull/727#discussion_r1537635981))
 
+#### longhands and shorthands
+
+Use the terms "shorthand" and "longhand" to describe the relationship between CSS properties that combine multiple properties into a single declaration and the individual properties. 
+
+Avoid using "shorthand" and "longhand" without the word "property":
+
+* Prefer "The `text-wrap` CSS property is a shorthand" over "The `text-wrap` CSS shorthand".
+* Prefer "It is a longhand property of" over "It is a longhand of".
+
+Avoid the phrase "constituent properties" for longhand properties, even though this is common on MDN Web Docs.
+([#1764](https://github.com/web-platform-dx/web-features/pull/1764#discussion_r1777335770))
+
 #### platform
 
 "platform" is often vague.