Something like this?
<!DOCTYPE html>
<html>
<head>
<style>
form * {
display: block;
margin: 10px;
}
</style>
<script language="Javascript" >
function download(filename, text) {
var pom = document.createElement('a');
pom.setAttribute('href', 'data:text/plain;charset=utf-8,' +
encodeURIComponent(text));
pom.setAttribute('download', filename);
pom.style.display = 'none';
document.body.appendChild(pom);
pom.click();
document.body.removeChild(pom);
}
</script>
</head>
<body>
<form onsubmit="download(this['name'].value, this['text'].value)">
<input type="text" name="name" value="test.txt">
<textarea rows=3 cols=50 name="text">Please type in this box. When you
click the Download button, the contents of this box will be downloaded to
your machine at the location you specify. Pretty nifty. </textarea>
<input type="submit" value="Download">
</form>
</body>
</html>