if the url contains mixed character sets, the browser will display the punycode version (punycode = non beautified, using only simple characters (ASCII), version of the url)
for example citibаnk.com is in fact displayed https://xn--citibnk-6fg.com/ in my browser
I agree that having a block at the url registration level would be better though. Maybe there are some use cases of mixed sets ?
cf "IDN Display Algorithm" on mozilla wiki, https://wiki.mozilla.org/IDN_Display_Algorithm