fix: remove margin reset (#3667)

Resetting the margin on all elements breaks some default styles like the
native dialog placement.

.changeset/shiny-ears-care.md

@@ -0,0 +1,5 @@
+---
+'@swisspost/design-system-styles': patch
+---
+
+Updated margin styles according to the contribution guidelines. Content elements like headings and paragraphs no longer have a margin top if they're the first child and no margin bottom if they're the last child in a container.

packages/styles/src/elements/heading.scss

@@ -3,20 +3,18 @@
 
 tokens.$default-map: elements.$post-heading;
 
-h1,
-.h1,
-h2,
-.h2,
-h3,
-.h3,
-h4,
-.h4,
-h5,
-.h5,
-h6,
-.h6 {
+// :is keeps specificity which is handy here
+:is(h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6) {
   line-height: tokens.get('heading-line-height');
   font-weight: tokens.get('heading-font-weight');
+
+  &:first-child {
+    margin-block-start: 0;
+  }
+
+  &:last-child {
+    margin-block-end: 0;
+  }
 }
 
 h1,

packages/styles/src/elements/reset.scss

@@ -9,20 +9,14 @@
   box-sizing: border-box;
 }
 
-/*
-  Remove default margin
-*/
-* {
-  margin: 0;
-}
-
 /*
   Typographic tweaks!
   - Add accessible line-height
   - Improve text rendering
 */
 body {
   -webkit-font-smoothing: antialiased;
+  margin: 0;
 }
 
 /*