) in html 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