Windows-1254 is a code page used under Microsoft Windows (and for the web), to write Turkish that it was designed for (and the vast majority of users use it for that language, even though it can also be used for some other languages).
Characters with codepoints A0 through FF are compatible with ISO 8859-9, but the CR range, which is reserved for C1 control codes in ISO 8859, is instead used for additional characters (analogous to the relationship between ISO-8859-1 and Windows-1252).
It matches Windows-1252 except for the replacement of six Icelandic characters (Ðð, Ýý, Þþ) with characters unique to the Turkish alphabet (Ğğ, İ, ı, Şş).
The WHATWG Encoding Standard, which specifies the character encodings which are permitted in HTML5 and which compliant browsers must support,[1] includes Windows-1254, which is used for both the Windows-1254 and ISO-8859-9 labels.
[2][3] Unicode is preferred for modern applications; authors of new pages and the designers of new protocols are instructed to use UTF-8 instead.