This tool allows you to normalise short pieces of text to Unicode forms NFC or NFD. You can paste the relevant text into a text area, or append it to the uri that calls the page, eg. Vietnamese example.
Note that, although I spell normalisation in the British way in this post, the uri uses the American spelling, since I suspect most users of the tool will expect it to be spelt that way.
Wondering what normalisation is? In Unicode a letter like á can be represented by a (precomposed) single character or by an a followed by an acute accent (a decomposed sequence). Unicode regards these two representations as formally equivalent. If you are comparing strings, therefore, you need to know which representations are equivalent. Usually you would want to normalise your text prior to comparison to a given normalisation form, so that the comparison process can be efficient. Unicode defines four normalization forms, two of which, NFD and NFC, are handled by this tool.
Basically NFD reduces all precomposed characters to their decomposed equivalents, whereas NFC uses precomposed characters for most common situations.