$('#details').toggle();

           if(document.getElementById('userName').userName.value=="")
           {
               alert("Name field cannot be empty");
               return false;
           }
           if (document.getElementById('userAge').value>=18)
           alert ("Name and Age are valid");
           return true;
       }

        <div class="container">
      
     
          <a class="navbar-brand" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23myPage"><i class="fas fa-dragon"></i> PROJECT</a>
      
       
          <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#basicExampleNav"
            aria-controls="basicExampleNav" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
          </button>
      
        
          <div class="collapse navbar-collapse" id="basicExampleNav">
      
        
            <ul class="navbar-nav mr-auto">
              <li class="nav-item active">
                <a class="nav-link" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23"><i class="fas fa-home"></i> Home
                  <span class="sr-only">(current)</span>
                </a>
              </li>
              <li class="nav-item">
                <a class="nav-link" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23me"><i class="fas fa-user"></i>ME</a>
              </li>
                <li class="nav-item">
                <a class="nav-link" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23movie"><i class="fas fa-film"></i>MOVIE</a>
              </li>
      
              
              <li class="nav-item dropdown">
                <a class="nav-link dropdown-toggle" id="navbarDropdownMenuLink" data-toggle="dropdown" aria-haspopup="true"
                  aria-expanded="false"><i class="fas fa-chevron-circle-down"></i> Dropdown</a>
                <div class="dropdown-menu dropdown-primary" aria-labelledby="navbarDropdownMenuLink">
                  <a class="dropdown-item" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23game"><i class="fas fa-gamepad"></i> GAMES</a>
                  <a class="dropdown-item" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23soccer"><i class="fas fa-futbol"></i> SOCCER</a>
                  <a class="dropdown-item" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23music"><i class="fas fa-music"></i> MUSIC</a>
                </div>
              </li>
      
            </ul>
           
      
            <form class="form-inline">
              <div class="md-form my-0">
                <input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search">
              </div>
            </form>
          </div>
         
      
        </div>
      
      </nav>

      <div id="intro" class="view">

        <div class="mask rgba-black-light" style="text-align: center;margin-top: 50px;">

           
            <h2 class="display-4 font-weight-bold white-text pt-5 mb-2"><i class="fas fa-heart red-text"></i> WELCOME<script style="color: yellow;">document.write("<h2> My Beloved "+username+"</h2>")</script><i class="fas fa-heart pink-text"></i></h2>

            <hr class="hr-light">

            <h4 class="white-text my-4">BLACK LIVES MATTER </h4>
            <div id="details" style="display: none;color: white;">

                <p>愿世间，再无流离失所；愿众生，再无谎言愚弄；愿天下，再无不公</p>
         
             </div>
            <button type="button" class="btn btn-outline-white" id="more">CLICK ME<i class="fas fa-book ml-2"></i></button>
          
            <br>

            <button type="button" class="btn btn-outline-danger" data-toggle="modal" data-target="#centralModalSuccess"><i class="fas fa-arrow-alt-circle-right"></i> CLICK ME  <i class="fas fa-arrow-alt-circle-left"></i>
           </button>
           
           
            <div class="modal fade" id="centralModalSuccess" tabindex="-1" role="dialog" aria-labelledby="myModalLabel"
              aria-hidden="true">
              <div class="modal-dialog modal-notify modal-danger" role="document">
                <!--Content-->
                <div class="modal-content">
                  <!--Header-->
                  <div class="modal-header">
                    <p class="heading lead">SURPRISE!!!</p>
           
                   
                  </div>
           
                  <!--Body-->
                  <div class="modal-body">
                    <div class="text-center">
                  
                      <img src="proxy.php?url=https%3A%2F%2Fi.pinimg.com%2Foriginals%2Ffd%2Fb0%2Fe0%2Ffdb0e0bc978358c1a82ee712f7141739.jpg" alt="hahaha" width="100%" height="500px">
                      <br>
                      <p>It is just a little nerve stimulation to wake reader's mind up~~</p>
                    </div>
                  </div>
           
             
                  <div class="modal-footer justify-content-center">
                    <a type="button" class="btn btn-danger" onclick="button1()">I LIKE IT<i class="far fa-gem ml-1 text-white"></i></a>
                    <a type="button" class="btn btn-outline-danger waves-effect" data-dismiss="modal">CLOSE</a>
                  </div>
                </div>
             
              </div>
            </div>
        </div>
      
      </div>

      <main class="mt-5">
     <div class="container">
       <div class="row d-flex justify-content-center mb-4">
       <div class="col-md-8">
         <h1 class="mb-5 font-weight-bold" style="margin-top: 50px;">Hello Welcome <i class="fas fa-heart pink-text"></i></h1>
         <p class="grey-text">Welcome to my project, hope you enjoy viewing my project and give high marks, as high as you can. Gracias...</p>
       </div>
       <div class="row">

       
        <div class="col-lg-4 col-md-4 mb-1">
          <i class="fas fa-home fa-4x blue-text"></i>
            <h4 class="my-4 font-weight-bold">Home</h4>
            <p class="grey-text">I have two siblings, my younger brother and sister, including my parents, this family is made of 5 members. I love my family. </p>
        </div>
      
        <div class="col-lg-4 col-md-4 mb-1">
            <i class="fas fa-guitar fa-4x brown-text"></i>
            <h4 class="my-4 font-weight-bold">Guitar</h4>
            <p class="grey-text">I love to play guitar. When I was stressed and exhausted, playing guitar helps to relieve my harassment</p>
        </div>
        

        <div class="col-lg-4 col-md-4 mb-1">
            <i class="fas fa-bicycle fa-4x purple-text"></i>
            <h4 class="my-4 font-weight-bold">My favourite Motivating Quotes</h4>
            <p class="grey-text">Do my Best, So that I can't blame myself for anything.<br>No Pain, No Gain<br>Don't Just Think, Just Do It.<br>Be Confident.Anything Is Possible.</p>
        </div>
       </div>
     </div>
     </main>

          <div id="carouselExampleControls" class="carousel slide" data-ride="carousel">
            <div class="carousel-inner">
              <div class="carousel-item active">
                <img class="d-block w-100" src="proxy.php?url=https%3A%2F%2Fwallpaperaccess.com%2Ffull%2F1526021.jpg"
                  alt="First slide">
              </div>
              <div class="carousel-item">
                <img class="d-block w-100" src="proxy.php?url=https%3A%2F%2Fwallpapercave.com%2Fwp%2FgJvyuLv.jpg"
                  alt="Second slide">
              </div>
              <div class="carousel-item">
                <img class="d-block w-100" src="proxy.php?url=https%3A%2F%2Fi.pinimg.com%2Foriginals%2F27%2F3e%2F43%2F273e43a71854d8359186ecc348370f8d.jpg"
                  alt="Third slide">
              </div>
            </div>
            <a class="carousel-control-prev" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23carouselExampleControls" role="button" data-slide="prev">
              <span class="carousel-control-prev-icon" aria-hidden="true"></span>
              <span class="sr-only">Previous</span>
            </a>
            <a class="carousel-control-next" href="proxy.php?url=https%3A%2F%2Fgithub.com%2F%23carouselExampleControls" role="button" data-slide="next">
              <span class="carousel-control-next-icon" aria-hidden="true"></span>
              <span class="sr-only">Next</span>
            </a>
          </div>