There is no guarantee that html() will be completely escaped so the result might not be safe after concatenation.
html() is based on innerHTML, and a browser could, without violating lots of expectations, implement innerHTML so that $("").text("1 <").html() is "1 <", and that $("").text("b>").html() is "b>".
Then if you concatenate those two individually safe results, you get "1 " which will obviously not be the HTML version of the concatenation of the two plaintext pieces.
So, this method is not safe by deduction from first principles, and there's no widely followed spec of innerHTML (though HTML5 does address it).
The best way to check if it does what you want is to test corner cases like this.
