三角形はborderの組み合わせで作成します。また、オブジェクトを1px動かして、枠線を消すテクニックを使います。
<!DOCTYPE html> <html lang="ja"> <head> <meta charset="utf-8"> <title>CSS Layout practice</title> <link rel="stylesheet" href="normalize.css"> <!-- <link rel="stylesheet" href="reset.css"> --> <link rel="stylesheet" href="styles.css"> <style> body { padding: 0 10px 40px; } h2 { font-size: 16px; } .ex1 { width: 100px; height: 100px; background: #eee; } .ex2 { width: 100px; height: 100px; background: #eee; border-style: solid; border-width: 20px; border-color: red blue pink green; } .ex3 { width: 200px; height: 100px; background: #eee; border-radius:3px; position: relative; } .ex3:before { position: absolute; right: 20px; bottom: -20px; content: ''; width: 0: height: 0; border-style: solid; border-width: 10px; border-color: #eee transparent transparent transparent; } .ex4 { width: 200px; height: 100px; background: #eee; border-radius:3px; position: relative; } .ex4:before { position: absolute; right: 20px; bottom: -20px; content: ''; width: 0: height: 0; border-style: solid; border-width: 10px 20px 10px 0; border-color: #eee transparent transparent transparent; } .ex5 { width: 200px; height: 100px; border: 1px solid #eee; border-radius:3px; position: relative; } .ex5:before { position: absolute; right: 20px; bottom: -20px; content: ''; width: 0: height: 0; border-style: solid; border-width: 10px; border-color: #eee transparent transparent transparent; } .ex5:after { position: absolute; right: 20px; bottom: -19px; content: ''; width: 0: height: 0; border-style: solid; border-width: 10px; border-color: white transparent transparent transparent; } .ex6 { width: 200px; height: 100px; border: 1px solid #eee; border-radius:3px; position: relative; } .ex6:before { position: absolute; right: 20px; bottom: -20px; content: ''; width: 0: height: 0; border-style: solid; border-width: 10px 20px 10px 0; border-color: #eee transparent transparent transparent; } .ex6:after { position: absolute; right: 20px; bottom: -19px; content: ''; width: 0: height: 0; border-style: solid; border-width: 10px 20px 10px 0; border-color: #fff transparent transparent transparent; } </style> </head> <body> <h2>ex1</h2> <div class="ex1"></div> <h2>ex2</h2> <div class="ex2"></div> <h2>ex3</h2> <div class="ex3"></div> <h2>ex4</h2> <div class="ex4"></div> <h2>ex5</h2> <div class="ex5"></div> <h2>ex6</h2> <div class="ex6"></div> </body> </html>