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One common approach is what's called Server Side Includes, or SSI. The language PHP also s another approach for includes.
These approaches are all server side, and require that your website's server is enabled for that technology, but such service is common these days.
A Google Search [google.com] limited to this very forum will give you about 300 results you can learn from about SSI.
Instead of naming your files as .htm or .html, name them as .shtml
Put the following line where you want the include to appear:
<!--#include file="blablabla.shtml" -->
So you .shtml would look like this:
<html>
<head><title></title>
</head>
<body>
blablabla, blablabla, blablabla, blablabla
<!--Now you want information from xyz.shtml to appear so you put in-->
<!--#include file="xyz.shtml" -->
</body>
</html>
Make sure YOU DO type in the comment marks. Enter the tag as shown and it will work. You won't be able to see it working while the page is hosted on your personnal computer(unless you have IIS installed and configured accordingly), but it will work as soon as you upload your pages on your webserver.
If you upload an external javascript, css or image file on your webspace you can reference that single file on an unlimited number of webpages.
But you can't do the same with code.
I wonder if there will ever be a way to have:
<code type="text/xhtml" src="/navlinks.html"></code>
in a normal xhtml document?