Create gradient-border-with-radius.md

Create gradient-border-with-radius snippet

snippets/gradient-border-with-radius.md

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+---
+title: Gradient border with radius
+tags: visual,intermediate
+---
+
+Makes a gradient border with radius.
+
+```html
+<div class="gradient-border"></div>
+```
+
+```css
+.gradient-border {
+  width: 300px;
+  height: 300px;
+  position: relative;
+  border: solid 1px transparent;
+  border-radius: 10px;
+  background-color: #f7f7fe;
+  background-clip: padding-box;
+}
+.gradient-border::before {
+  content: '';
+  position: absolute;
+  top: 0;
+  right: 0;
+  bottom: 0;
+  left: 0;
+  z-index: -1;
+  margin: -1px;
+  border-radius: inherit;
+  background: linear-gradient(to bottom, #d3e7ec, #fff);
+}
+```
+
+#### Explanation
+
+- Create a block with a transparent border, relative position and some background.
+- Make a absolutely positioned substrate with `::before` pseudo-element with gradient background and `z-index: -1`.
+- Use `top: 0`, `right: 0`, `bottom: 0`, `left: 0` to make the substrate equal to block.
+- Negate the substrate height and width using `margin: -1px` to make substrate bigger then block.
+- Use `background-clip: padding-box` to not draw block's background below the border.
+
+#### Browser support
+
+- https://caniuse.com/#feat=mdn-css_properties_background_background-clip