To-do list - HTML/CSS/JS
so just 2 things here:


if(task=="") alert("Are you sure you do not have any task to do?");
such comparison is a bit risky in JS. In case of checking for empty values it's better to do if(!condition) {} else {} or at least if(task ==="") (strong type checking). Not saying it's wrong in this case, just a rule of a thumb. Read more about issues with JS comparison / type casting, you'll find out what I mean.
Creating UI in code is fine of course, and it's nice to understand what's going on under the hood, but for readability of your UI you mind find it better to define everything up-front in HTML and than later just show / hide parts of it or add remove classes, depending on your application state. This is not a general guideline, because it only makes sense for small-medium apps, for larger, more complex, you probably will use some framework anyway (angular or such).
Other than that, fine simple app (assuming it works, didn't run it to be honest) ;).