--- /dev/null
+# function-linear-gradient-no-nonstandard-direction
+
+Disallow direction values in `linear-gradient()` calls that are not valid according to the
+[standard syntax](https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient#Syntax).
+
+```css
+.foo { background: linear-gradient(to top, #fff, #000); }
+/** ↑
+ * This (optional) first argument is the "direction" */
+```
+
+A valid and standard direction value is one of the following:
+
+- an angle
+- `to ` plus a side-or-corner (`to top`, `to bottom`, `to left`, `to right`; `to top right`, `to right top`, `to bottom left`, etc.)
+
+A common mistake (matching outdated non-standard syntax) is to use just a side-or-corner without the preceding `to`.
+
+## Options
+
+### `true`
+
+The following patterns are considered violations:
+
+```css
+.foo { background: linear-gradient(top, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(bottom, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(left, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(45, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(to top top, #fff, #000); }
+```
+
+The following patterns are *not* considered violations:
+
+```css
+.foo { background: linear-gradient(to top, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(to bottom right, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(45deg, #fff, #000); }
+```
+
+```css
+.foo { background: linear-gradient(1.57rad, #fff, #000); }
+```
+
+```css
+/* Direction defaults to "to bottom" */
+.foo { background: linear-gradient(#fff, #000); }
+```