ASCII, standardised in 1963, encodes text using 7 bits per character — enough for 128 slots. That covers the English alphabet in both cases, digits, punctuation and control codes, and nothing else. No é, no ñ, no ₹, certainly no 𝗯𝗼𝗹𝗱 alphabets.
The 8-bit chaos
As computing spread, vendors crammed extra characters into the eighth bit, producing dozens of incompatible "extended ASCII" code pages. A file written in the Greek code page displayed as gibberish on a machine using the Cyrillic one. International text exchange was fundamentally broken.
Unicode's answer
Unicode, first published in 1991, gave every character its own permanent number in a single space of over a million code points. Crucially, its dominant encoding — UTF-8 — was designed so the first 128 code points are byte-for-byte identical to ASCII. Every valid ASCII file is already valid UTF-8, which let the web migrate gradually; today UTF-8 carries over 98% of web pages.
Why it matters for text styles
Everything this site generates lives far outside ASCII's 128 characters — mathematical alphabets sit around code point 120,000. When a platform says it accepts "plain text", it almost always means Unicode text, which is why 𝕗𝕒𝕟𝕔𝕪 letters can travel anywhere ordinary text can. ASCII text is the lowest common denominator; Unicode is the actual language of the modern web.