add preview for markup

pdb-js/src/styles.scss

@@ -223,8 +223,27 @@ a.external-link:after {
 
 /* styles for markdown*/
 markdown blockquote {
-  border-left: 4px grey solid;
+  border-left: 3px grey solid;
   display: block;
-  margin-left: 2em;
-  padding-left: 0.5em;
+  margin: 1em 0 1em 2em;
+  padding: 0.5em 0 0.5em 0.5em;
+}
+
+markdown table {
+  border-collapse: collapse;
+}
+
+markdown table th, markdown table td {
+  border: solid 1px black;
+  padding: 0.4em;
+}
+markdown thead {
+  border-bottom: 2px black solid;
+}
+markdown tfoot {
+  border-top: 2px black solid;
+}
+
+markdown pre {
+  font-family: monospace;
 }