No. ASCII is a subset of UTF-8, ISO-8859-1 is not. The first 256 codepoints of unicode match ISO-8859-1, which is probably the source of your confusion. However

No. ASCII is a subset of UTF-8, ISO-8859-1 is not. The first 256 codepoints of unicode match ISO-8859-1, which is probably the source of your confusion. However Codepoints 128-255 are encoded differently in UTF-8. They are represented by a single byte when encoded as ISO-8859-1, while they turn into two bytes encoded in UTF-8. Plus "ISO-8859-1" is treated as Windows-1252 by browsers, while unicode uses ISO-8859-1 extended with the ISO 6429 control characters for its initial 256 codepoints.