feat: Inverted corners

contents/_includes/patterns/inverted-corners.njk

@@ -0,0 +1,2 @@
+<div class="inverted-corners">
+</div>

contents/index.njk

@@ -163,6 +163,12 @@ eleventyExcludeFromCollections: true
                     <div class="pattern__title">Initial avatar</div>
                 </a>
             </div>
+            <div class="pattern__item">
+                <a class="pattern__link" href="/inverted-corners/">
+                    <div class="pattern__cover">{% include "patterns/inverted-corners.njk" %}</div>
+                    <div class="pattern__title">Inverted corners</div>
+                </a>
+            </div>
             <div class="pattern__item">
                 <a class="pattern__link" href="/triangle-buttons/">
                     <div class="pattern__cover">{% include "patterns/triangle-buttons.njk" %}</div>

contents/inverted-corners.md

@@ -0,0 +1,52 @@
+---
+layout: layouts/post.njk
+title: Inverted corners
+description: Create inverted corners with CSS
+keywords: css border radius, css inverted border radius, css inverted corners
+---
+
+## HTML
+
+```html
+<div class="inverted-corners">
+    ...
+</div>
+```
+
+## CSS
+
+```css
+:root {
+    --inverted-corners-background: #d1d5db;
+    --inverted-corners-size: 2rem;
+}
+
+.inverted-corners {
+    background-color: var(--inverted-corners-background);
+
+    /* Used to position the corner */
+    position: relative;
+}
+
+.inverted-corners::before {
+    content: '';
+
+    /* Absolute position */
+    bottom: calc(-2 * var(--inverted-corners-size));
+    left: 0;
+    position: absolute;
+
+    /* Size */
+    height: calc(2 * var(--inverted-corners-size));
+    width: var(--inverted-corners-size);
+
+    /* Border */
+    background-color: transparent;
+    border-top-left-radius: var(--inverted-corners-size);
+    box-shadow: var(--inverted-corners-background) 0px calc(-1 * var(--inverted-corners-size)) 0px 0px;
+}
+```
+
+{% demo %}
+{% include "patterns/inverted-corners.njk" %}
+{% enddemo %}