Rendering software which cannot process a Unicode character appropriately often displays it as an open rectangle, or the Unicode "replacement character" (U+FFFD, ), to indicate the position of the unrecognized character. Some systems have made attempts to provide more information about such characters. The Apple's Last Resort font will display a substitute glyph indicating the Unicode range of the character, and the SIL International's Unicode Fallback font will display a box showing the hexadecimal scalar value of the character.
What does SIL's Unicode Fallback font display when it can't display a character properly?
a box showing the hexadecimal scalar value of the character

Around the sixth century AD, a tribe of Slavs arrived in a portion of Central Europe. According to legend they were led by a hero named Cech, from whom the word "Czech" derives. The ninth century brought the state of Great Moravia, whose first ruler (Rastislav of Moravia) invited Byzantine ruler Michael III to send missionaries in an attempt to reduce the influence of East Francia on religious and political life in his country. These missionaries, Constantine and Methodius, helped to convert the Czechs from traditional Slavic paganism to Christianity and established a church system. They also brought the Glagolitic alphabet to the West Slavs, whose language was previously unwritten. This language, later known as Proto-Czech, was beginning to separate from its fellow West Slavic hatchlings Proto-Slovak, Proto-Polish and Proto-Sorbian. Among other features, Proto-Czech was marked by its ephemeral use of the voiced velar fricative consonant (//) and consistent stress on the first syllable.
Whose influence was Rastislav eager to reduce, when he invited Michael III to send missionaries?
East Francia

Notable exceptions include the massacre of Jews and forcible conversion of some Jews by the rulers of the Almohad dynasty in Al-Andalus in the 12th century, as well as in Islamic Persia, and the forced confinement of Moroccan Jews to walled quarters known as mellahs beginning from the 15th century and especially in the early 19th century. In modern times, it has become commonplace for standard antisemitic themes to be conflated with anti-Zionist publications and pronouncements of Islamic movements such as Hezbollah and Hamas, in the pronouncements of various agencies of the Islamic Republic of Iran, and even in the newspapers and other publications of Turkish Refah Partisi."
What were walled quarters known as?
mellahs