adding HTML highlight block rule
diff --git a/user_guide_src/source/_themes/eldocs/static/asset/css/common.css b/user_guide_src/source/_themes/eldocs/static/asset/css/common.css
index 10e4de1..d1e9fe2 100644
--- a/user_guide_src/source/_themes/eldocs/static/asset/css/common.css
+++ b/user_guide_src/source/_themes/eldocs/static/asset/css/common.css
@@ -157,6 +157,7 @@
 .highlight-bash,
 .highlight-perl,
 .highlight-php,
+.highlight-html,
 .cp-path,
 .codeblock{
 	background-color: #F9FEFF;