Type in some text and press Convert
This works for upper case, lower case, numbers and some punctuation.
You can also convert already upside-down text back to right-way-up (use copy and paste).
To flip text upside-down using CSS, just apply the rule transform: rotate(180deg);
To reverse text (right to left) using CSS, apply the rule transform: scale(-1, 1);
Flip text upside down using CSS Transform