Got errors when using XMLHttpRequest

Question

I have problems when using XMLHttpRequest. One occurs in Chrome; the other in firefox

xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
    if (this.readyState == 4) {
        if (this.status == 200) 
            elmnt.innerHTML = this.responseText;
        }
        if (this.status == 404) {
            elmnt.innerHTML = "Page not found.";
        }
    }
}
xhttp.open("GET", file, true);
xhttp.send();

I'm working locally, i.e. all protocols are file://

In chrome, where I have added parameter --allow-file-access-from-files, this.status is 0. Why?

In firefox, the status is 200, but there will be a XML parsing error when the included file has code like this:

<script>
alert(-1<1);
</script>

and an XML parsing syntax error occurred indicating 10th character.

Why? I guess firefox does not recognize '<' between script.

Are they bugs or some issues I don't know yet?