Код CSS для создания таких градиент
        - .container{
 width: 400px;
 height: 300px;
 margin-top: 100px;
 margin-left: 150px;
 background-color: yellow;
 background: -webkit-linear-gradient(top right, black, red, yellow  );
 background: -moz-linear-gradient(top right, black, red, yellow  );
 background: -o-linear-gradient(top right, black, red, yellow  );
 background: -ms-linear-gradient(top right, black, red, yellow  );
 }
 
-  .container{
 width: 400px;
 height: 300px;
 margin-top: 100px;
 margin-left: 150px;
 background: -webkit-radial-gradient(center, black, red, yellow  );
 background: -moz-radial-gradient(center, black, red, yellow  );
 background: -o-radial-gradient(center, black, red, yellow  );
 background: -ms-radial-gradient(center, black, red, yellow  );
 }
 
 
     
    
            
                Код HTML для создания таких градиент
                    <!DOCTYPE html>
                    <html lang="ru">
                    <head>
                    <meta charset="UTF-8">
                    <title>CSS</title>
                    Добавление языка мэрри вэзэр
                    <link href="https://fonts.googleapis.com/css?family=Merriweather|PT+Sans
+Caption&display=swap" rel="stylesheet">
                    <link rel="stylesheet" href="../css/lessons1_css.css">
                    <link rel="shortcut icon" type="image/x-icon" href="../img/lamp.jpg">
                    <div class="lesson1"><a href="CSS_page.html" 
class="button-back">← CSS lessons</a></div>
                    </head>
                    <body>
                    <div class="container"></div>
                    </body>
 
                    </html>