span.highlight{background:yellow; padding:3px;}

<input type="search"/>
<p>The Ama...</p>

var s = document.querySelector('input[type="search"]'),
    p = document.querySelector('p'),
    find = function(){
    var words = p.innerText.split(' ');
    for(var i=0; i<words.length; i++){
        if(words[i].toLowerCase() == s.value.toLowerCase()){
            words[i] = '<span class="highlight">' + words[i] + "</span>";
            p.innerHTML = words.join(' ');
        }
        else{

        }
    }
}
s.addEventListener('keydown', find , false);
s.addEventListener('keyup', find , false);

var s = document.querySelector('input[type="search"]'),
    p = document.querySelector('p'),
    find = function(){
        var words = p.innerText.split(' '),
            i = words.length,
            word = '';

        while(--i) {
            word = words[i];
            if(word.toLowerCase() == s.value.toLowerCase()){
                words[i] = '<span class="highlight">' + word + "</span>";
            }
            else{

            }
        }

        p.innerHTML = words.join(' ');
    }

s.addEventListener('keydown', find , false);
s.addEventListener('keyup', find , false);