[html] Apply CSS rules to a nested class inside a div

I don’t know exactly how to apply CSS to a nested element. Here is my example code, but I’m looking for a manual that explains all the rules:

<div id="content">
  <div id="main_text">
    <h2 class="title"></h2>
  </div>
</div>

How can I apply CSS to only the class title, nested inside that particular div?

Use Css Selector for this, or learn more about Css Selector just go here https://www.w3schools.com/cssref/css_selectors.asp

#main_text > .title {
  /* Style goes here */
}

#main_text .title {
  /* Style goes here */
}

If you need to target multiple classes use:

#main_text .title, #main_text .title2 {
  /* Properties */
}