def to_css_length(l):
if isinstance(l, (int, float)):<EOL><INDENT>return '<STR_LIT>'.format(l)<EOL><DEDENT>else:<EOL><INDENT>return l<EOL><DEDENT>
Return the standard length string of css.
It's compatible with number values in old versions.
:param l: source css length.
:return: A string.