Replace Sass with PostCSS, include imports and nesting

assets/css/base/base.css

@@ -0,0 +1,43 @@
+a {
+    color: inherit;
+    text-decoration: none;
+
+    &:focus,
+    &:hover {
+        text-decoration: underline;
+    }
+}
+
+main a {
+    text-decoration: underline;
+
+    &:focus,
+    &:hover {
+        text-decoration: none;
+    }
+}
+
+h1, h2, h3 {
+    @apply mb-2;
+}
+
+.content h2,
+.content h3 {
+    @apply mt-6;
+}
+
+p,
+ul,
+ol,
+blockquote,
+table,
+pre {
+    @apply mb-6;
+}
+
+.element-invisible {
+    @apply absolute overflow-hidden break-normal;
+    clip: rect(1px, 1px, 1px, 1px);
+    height: 1px;
+    width: 1px;
+}