Reference
The technical side: the Unicode blocks each style draws from, what they were encoded for, and why the gaps in them are where every missing letter comes from.
Reference pages
Concepts
What a Unicode Code Point Is, and Why It Is Not a ByteA Unicode code point explained: what U+0041 means, how code points differ from bytes and characters, and why a five-letter word can be twenty characters long.
UTF-8 vs Unicode: The Difference, and Why It MattersUTF-8 vs Unicode explained: Unicode assigns numbers to characters, UTF-8 turns those numbers into bytes, and mixing them up is what produces mojibake.
Unicode Normalization: NFC, NFD, NFKC and NFKD ExplainedUnicode normalization forms compared: what NFC, NFD, NFKC and NFKD each do, which one to use for storage and search, and why NFKC destroys styled text.
Combining Diacritical Marks: The Blocks and How They StackCombining diacritical marks in Unicode: the four blocks, how a mark attaches to a base character, the stacking rules, and what underline and Zalgo actually use.
Character blocks
Unicode Blocks Behind Every Style This Site GeneratesWhich Unicode block each of the 32 styles draws from, with the exact code point ranges, the official charts, and the characters each block leaves out.
Mathematical Alphanumeric Symbols: The Block Behind It AllThe Mathematical Alphanumeric Symbols block, U+1D400 to U+1D7FF: every styled alphabet it contains, the sub-ranges, and the holes where letters are missing.
Circled Letters in Unicode: Bubbles, Squares and Their GapsCircled letters and squared letters in Unicode: the Enclosed Alphanumerics blocks, the full ranges, and which variants have no lowercase or no digits.
Fullwidth Text: The Halfwidth and Fullwidth Forms BlockFullwidth text explained: what the Halfwidth and Fullwidth Forms block is, why the characters are double width, what it covers, and why NFKC removes it.
Small Caps in Unicode: Where the Letters Come FromSmall caps Unicode reference: the phonetic blocks the small capital letters live in, the full alphabet, the letters that do not exist, and why X is missing.
Superscript and Subscript Letters: The Full Unicode ListEvery superscript and subscript letter and digit in Unicode, which blocks they live in, and the exact letters that do not exist in either alphabet.
Zero Width Space and the Other Invisible Unicode CharactersThe zero width space U+200B and its relatives: what each invisible character is for, which survive a paste, how to find them, and how to strip them from text.
Related
Every style page cites the blocks its letters come from and links the official chart. The guide covers the same ground in plainer language, and the blog has the longer pieces.