What is the best way to target elements in a DIV?

Question

I have some styles that I want to use in SharePoint but it always messes up my SharePoint page by making things appear weird. I can't even use something simple.

How can I use CSS to target just the elements inside of a div (myDiv below) that has an ID?

<h3>Using CSS to style an HTML Form</h3>

<div id="myDiv">
  <form action="/action_page.php">
    <label for="fname">First Name</label>
    <input type="text" id="fname" name="firstname" placeholder="Your name..">

    <label for="lname">Last Name</label>
    <input type="text" id="lname" name="lastname" placeholder="Your last name..">

    <label for="country">Country</label>
    <select id="country" name="country">
      <option value="australia">Australia</option>
      <option value="canada">Canada</option>
      <option value="usa">USA</option>
    </select>

    <input type="submit" value="Submit">
  </form>
</div>

  <form action="/action_page.php">
    <label for="fname">First Name</label>
    <input type="text" id="fname" name="firstname" placeholder="Your name..">

    <label for="lname">Last Name</label>
    <input type="text" id="lname" name="lastname" placeholder="Your last name..">

    <label for="country">Country</label>
    <select id="country" name="country">
      <option value="australia">Australia</option>
      <option value="canada">Canada</option>
      <option value="usa">USA</option>
    </select>

    <input type="submit" value="Submit">
  </form>

For more, please see codepen

Answer 1

You can do it like below :

<style type="text/css">
  #myDiv
  {
    // your CSS styles
  }
  #fname
  {
    // your CSS styles
  }
  #lname
  {
    // your CSS styles
  }
  #country
  {
    // your CSS styles
  }
 </style>

Answer 2

In the div, it will have 2 inputboxes and 1 select dropdown which have Id, you could define CSS based on type:

<style type="text/css">
input[type=text], select {
  width: 100%;
  padding: 12px 20px;
  margin: 8px 0;
  display: inline-block;
  border: 1px solid #ccc;
  border-radius: 4px;
  box-sizing: border-box;
}

div {
  border-radius: 5px;
  background-color: #f2f2f2;
  padding: 20px;
}
</style>

Style HTML Input Forms