html reserved tags code example

Example 1: . and # in html

<!DOCTYPE html>
<html>
<head>
<style>
.classname {
  background-color: green;
  color: white;
}
#idname {
  background-color: pink;
  color: white;
}
</style>
</head>
<body>
<div class="classname">I am  green colour<div>
<div id="idname">I am  pink colour</div>
</body>
</html>

Example 2: # in html

# in CSS means your looking at an ID so 
<div class="ExampleClass" id="ExampleID"></div>

#ExampleID would reference the ID
.ExampleClass would reference the class