Character encoding
The content of an HTML document is a sequence of characters and markup. To send it "over the wire", it is encoded as a sequence of bytes, using one of several possible encodings. The HTML document has to be decoded to find the characters. For example, in Western Europe it is customary to use the byte 224 for an a-with-grave-accent (à), but in Hebrew, it is more common to use 224 for an Aleph. In Japanese, the meaning of a byte usually depends on the bytes that preceded it. In some encodings, one character is encoded as two (or more) bytes.
The UA knows how to decode the bytes by looking at the "charset" parameter in the HTTP header. Typical encodings (charset values) are "ASCII" (for English), "ISO-8859-1" (for Western Europe), "ISO-8859-8" (for Hebrew), "Shift-JIS" (for Japanese).
HTML [2],[4], allows some 30,000 different characters, namely those defined by Unicode. Not many documents will use that many different characters, and choosing the right encoding will usually ensure that the document only needs one byte per character. Occasional characters outside the encoded range can still be entered as numerical character references: 'Π' will always mean the Greek uppercase Pi, no matter what encoding was used. Note that this entails that UAs have to be prepared for any Unicode character, even if they only handle a few encodings.
