There is no standard entity defined in HTML to represent the character ž (U + 017E) ; you can only use a digital link, for example ž (hexadecimal) or ž (decimal).
But you can define such an object for your document, for example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd" [ <!ENTITY Zcaron CDATA "Ž" -- latin capital letter Z with caron, U+017D ISOlat2 --> <!ENTITY zcaron CDATA "ž" -- latin small letter z with caron, U+017E ISOlat2 --> ]>
Now you can reference Zcaron objects (representing Ž ) and zcaron (representing ž ) with Ž and Ž respectively.
Or if you use a character set containing this character (for example, a Unicode character set), you can use a suitable character encoding (for example, UTF-8 in the case of Unicode) to encode that character directly instead of using a symbolic link.
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