用一个array来存放待删除的元素，就不需要跟下标打交道了

var webTech = ['JavaScript', 'HTML', 'CSS'];
var toDelete = [];
var ul = document.getElementById('test-list');

// find element to delete
for (var i=0; i<ul.children.length; i++){

    if(webTech.indexOf(ul.children[i].innerHTML) < 0){

        toDelete.push(ul.children[i]);
    }

}

// delete them
for (var node of toDelete){

    ul.removeChild(node);

}