Remove <mark> border because text nodes remain split.

A motivation to merge split nodes again in the removeHighlight function,
e.g. using Node.normalize().
diff --git a/demo/index.html b/demo/index.html
index facccec..0ae10c1 100644
--- a/demo/index.html
+++ b/demo/index.html
@@ -33,7 +33,6 @@
     }
     mark {
       background-color: rgba(255, 255, 0, 0.5);
-      outline: 0.1px solid rgba(255, 100, 0, 0.8);
     }
     #parsed {
       height: 15rem;