twitter-global-site-search-form.html

<div role="search">
    <form class="t1-form form-search js-search-form" action="/search" id="global-nav-search">
        <label class="visuallyhidden" for="search-query">Search query</label>
        <input class="search-input" type="text" id="search-query" placeholder="Search Twitter" 
        name="q" autocomplete="off" spellcheck="false" aria-autocomplete="list" 
        aria-expanded="false" aria-owns="typeahead-dropdown-7" dir="ltr">
        <span class="search-icon js-search-action">
            <button type="submit" class="Icon Icon--search nav-search" tabindex="-1">
                <span class="visuallyhidden">Search Twitter</span>
            </button>
        </span>
    </form>
</div>