preview.html

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Country Flags</title>
    <style>
        ul {
            list-style: none;
            padding: 0;
            margin: 0;
            width: 80%;
            max-width: 500px;
            display: flex;
            flex-wrap: wrap;
        }
        li {
            padding: 0;
            margin: 5px 5px 0 0;
        }
    </style>
</head>
<body>
    <div id="app"></div>
    <script>
        class CountriesPreview {
            constructor() {
                this.$el = document.querySelector('#app');
                this.build();
            }

            async getCountries() {
                const response = await fetch('countries.json');
                return await response.json();
            }

            async build() {
                const countries = await this.getCountries();
                const list = countries.map(country => {
                    return `<li><img src="images/svg/${country.alpha3.toLowerCase()}.svg" width="40" alt="${country.name}" /></li>`;
                });

                this.$el.innerHTML = `<ul>${list.join('\n')}</ul>`;
            }
        }

        new CountriesPreview();
    </script>
</body>
</html>