HTML Entities and/or ISO Latin-1 codes can be placed in source code like any other alphanumeric characters to produce special characters and symbols that cannot be generated in HTML with normal keyboard commands.
For example, to render Düsseldorf the HTML source should read
Düsseldorf or Düsseldorf
While many similar lists are available on the Web (run your favorite search engine using “ISO Latin” or “HTML Entities”), none I’ve seen account for the standard character sets of different operating systems (e.g. Windows vs. DOS vs. Macintosh, etc.); this list should produce the same results on all platforms.