You can just use the Blob function:
function save() {
var htmlContent = ["your-content-here"];
var bl = new Blob(htmlContent, {type: "text/html"});
var a = document.createElement("a");
a.href = URL.createObjectURL(bl);
a.download = "your-download-name-here.html";
a.hidden = true;
document.body.appendChild(a);
a.innerHTML = "something random - nobody will see this, it doesn't matter what you put here";
a.click();
}
and your file will save.