Add protocol whitelist for href/src attributes

- Add strict URL checker for src attributes
- Add protocol whitelist for href attributes
- Add error styling if URL fails to pass filter
- Add doc for new htconfig value
- Add emphasis to config key names in htconfig doc

view/global.css

@@ -365,15 +365,15 @@ a {
   color: #00a700;
 }
 .federation-graph {
-  width: 400px; 
-  height: 400px; 
-  float: right; 
+  width: 400px;
+  height: 400px;
+  float: right;
   margin: 20px;
 }
 .federation-network-graph {
-  width: 240px; 
-  height: 240px; 
-  float: left; 
+  width: 240px;
+  height: 240px;
+  float: left;
   margin: 20px;
 }
 ul.federation-stats,
@@ -429,7 +429,7 @@ td.federation-data {
 }
 
 .p-addr {
-  clear: both;	
+  clear: both;
 }
 
 #live-community {
@@ -481,3 +481,10 @@ td.pendingnote > p > span {
 	border-left: 5px solid #f00;
 	font-weight: bold;
 }
+
+/* src/href attributes filter error display */
+.invalid-src { border: 1px dotted red;}
+.invalid-href { border-bottom: 1px dotted red;}
+.invalid-src:after,
+.invalid-href:after { content: '⚠️'}
+img.invalid-src:after { vertical-align: top;}