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About the <head>

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CSS

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JavaScript

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HTML 

              
                <!DOCTYPE html>
<html>

<head>
  <script src="https://cdn.freecodecamp.org/testable-projects-fcc/v1/bundle.js"></script>
  <title> Technical Documentation Page </title>
</head>

<body>
  <nav id="navbar">
    <header>
      <h3> JS documentation </h3>
    </header>
    <ul>
      <a href=#Introduction class="nav-link">
        <li> Introduction </li>
      </a>
      <a href=#What_you_should_already_know class="nav-link">
        <li> What you should already know </li>
      </a>
      <a href=#JavaScript_and_Java class="nav-link">
        <li> JavaScript and Java </li>
      </a>
      <a href=#Hello_World class="nav-link">
        <li> Hello World </li>
      </a>
      <a href=#declaring_variables class="nav-link">
        <li> Declaring variables </li>
      </a>
      <a href=#variable_scope class="nav-link">
        <li> Variable Scope </li>
      </a>
    </ul>
  </nav>

  <main id="main-doc">
    <section id="Introduction" class="main-section">
      <header id="header">
        <h2> Introduction </h2>
      </header>
      <p> JavaScript is a cross-platform, object-oriented scripting language. It is a small and lightweight language. Inside a host environment (for example, a web browser), JavaScript can be connected to the objects of its environment to provide programmatic
        control over them. </p>

      <p> JavaScript contains a standard library of objects, such as Array, Date, and Math, and a core set of language elements such as operators, control structures, and statements. Core JavaScript can be extended for a variety of purposes by supplementing
        it with additional objects; for example: </p>
      <ul>
        <li>
          <p> Client-side JavaScript extends the core language by supplying objects to control a browser and its Document Object Model (DOM). For example, client-side extensions allow an application to place elements on an HTML form and respond to user events
            such as mouse clicks, form input, and page navigation. </li>
        </p>
        <li>
          <p> Server-side JavaScript extends the core language by supplying objects relevant to running JavaScript on a server. For example, server-side extensions allow an application to communicate with a database, provide continuity of information from
            one invocation to another of the application, or perform file manipulations on a server. </li>
        </p>
      </ul>
    </section>
    <section class="main-section" id="What_you_should_already_know">
      <header id="header">
        <h2> What you should already know </h2>
      </header>
      <p> This guide assumes you have the following basic background: </p>
      <ul>
        <li>
          <p> A general understanding of the Internet and the World Wide Web (WWW). </p>
        </li>
        <li>
          <p> Good working knowledge of HyperText Markup Language (HTML). 
          </p>
        </li>
        <li>
          <p> Some programming experience. If you are new to programming, try one of the tutorials linked on the main page about JavaScript. </p>
        </li>
      </ul>
    </section>
  
    <section class="main-section" id="JavaScript_and_Java">
      <header id="header">
        <h2> JavaScript and Java </h2>
      </header>
      <p> JavaScript and Java are similar in some ways but fundamentally different in some others. The JavaScript language resembles Java but does not have Java's static typing and strong type checking. JavaScript follows most Java expression syntax, naming
        conventions and basic control-flow constructs which was the reason why it was renamed from LiveScript to JavaScript. </p>

      <p> In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model
        instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements.
        Functions can be properties of objects, executing as loosely typed methods. </p>

      <p> JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces.
        Variables, parameters, and function return types are not explicitly typed. </p>
    </section>
    <section class="main-section" id="Hello_World">
      <header id="header">
        <h2> Hello World </h2>
      </header>
      <p> To get started with writing JavaScript, open the Scratchpad and write your first "Hello world" JavaScript code:
        </p>
          <code> <p> function greetMe(yourName) { alert("Hello " + yourName); }
             greetMe("World"); </p> </code>
          <p> Select the code in the pad and hit Ctrl+R to watch it unfold in your browser! </p>
    </section>
    <section class="main-section" id="declaring_variables">
      <header id="header">
        <h2> Declaring Variables </h2>
      </header>
      <p> You can declare a variable in three ways: With the keyword var. For example, </p>
      <code> var x = 42. </code>
      <p> This syntax can be used to declare both local and global variables. By simply assigning it a value. For example, </p>
      <code> x = 42. </code>
      <p> This always declares a global variable. It generates a strict JavaScript warning. You shouldn't use this variant. With the keyword let. For example, </p>
      <code>  let y = 13.  </code>
      <p> This syntax can be used to declare a block scope local variable. See Variable scope below. </p>
    </section>
    <section class="main-section" id="variable_scope">
      <header id="header">
        <h2> Variable Scope </h2>
      </header>
      <p> When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available
        only within that function. JavaScript before ECMAScript 2015 does not have block statement scope; rather, a variable declared within a block is local to the function (or global scope) that the block resides within. For example the following code
        will log 5, because the scope of x is the function (or global context) within which x is declared, not the block, which in this case is an if statement. </p>
      <code> if (true) { var x = 5; } console.log(x); // 5 </code>
      <p> This behavior changes, when using the let declaration introduced in ECMAScript 2015. </p>
      <code> if (true) { let y = 5; } console.log(y); // ReferenceError: y is not
defined </code>
    </section>

  </main>
</body>

<footer>
</footer>

</html>
!

CSS 

              
                #navbar {
  dispay: flex;
  flex-direction: column;
  justify-content: center;
  allign-items: center;
  width: 300px;
  position: fixed;
  z-index: 1;
  border-right: solid;
  border-color: rgba(0, 22, 22, 0.4);
  top: 0px;
  bottom: 0px; 
  height: 100%;
}

#navbar ul { 
  padding-right: 5px; 
  padding-left: 0;
  font-family: "Open Sans", Arial, Sans-serif;
  overflow-y: scroll;
    overflow-x: hidden;
  height: 70vh;
}

#navbar li {
  color: #4d4e53;
    border-top: 1px solid;
    list-style: none;
    position: relative;
  margin-bottom: 0;
  line-height: 90px;
}

header h3 {
  font-family: "Open Sans", Arial, Sans-serif;
  font-size: 30px;
  margin: 10px;
  text-align: center;
}

.nav-link {
  dispay: flex;
  flex-direction: column;
  justify-content: center;
  allign-items: center;
  list-style-type: none;
  text-decoration: none;
  color: rgb(77, 78, 83);
  cursor: pointer;
  line-height: 24px;
}

.main-section {
  display: flex;
  flex-direction: column;
  justify-content: center;
  text-align: center;
  margin-left: 300px;
  margin-bottom: 50px;
  padding: 30px;
}

#header h2 {
  text-align: start;
  margin: 0;
  font-family: "Open Sans", Arial, Sans-serif;
}

.main-section p {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  font-family: "Open Sans", Arial, Sans-serif;
  margin: 15px;
  text-align: start;
  line-height: 24px;
  padding: 0; 
}

@media only screen and (max-width: 600px) {
#navbar {
  display: flex;
  flex-direction: row;
    background-color: white;
    position: absolute;
    top: 0;
    padding: 0;
    margin: 0;
    width: auto;
  max-height: 30vh;
    border: none;
    z-index: 1;
    border-bottom: 2px solid;
  overflow-y: hidden;
}
}

@media only screen and (max-width: 600px) {
#navbar ul {
   padding-right: 5px; 
  padding-left: 0;
  padding-bottom: 0;
  font-family: "Open Sans", Arial, Sans-serif;
  overflow-y: scroll;
    overflow-x: hidden;
  height: 30vh;
  }
}
  
  
@media only screen and (max-width: 600px){
#Introduction {
    position: relative;
    margin-left: 0px;
    margin-top: 200px;
   display: flex;
  flex-direction: column;
  justify-content: center;
  text-align: center;
  }
}

@media only screen and (max-width: 600px){
.main-section {
    position: relative;
    margin-left: 0px;
    margin-top: 5px;
   display: flex;
  flex-direction: column;
  justify-content: center;
  text-align: center;
  }
}


code {
  display: flex;
  flex-direction: column;
  background-color: rgb(247, 247, 247);
  margin: 0;
  padding: 15px;
}
!

JS
!
999px

Console