HTML codebase Attribute

Introduction

The codebase attribute in HTML is used with the <applet> and <object> elements to specify the base URL for resources associated with an embedded application or plugin. It tells the browser where to fetch supplementary files—such as Java class files, JAR archives, or other assets—that the embedded content needs. This attribute was common in legacy pages that used Java applets and is now largely obsolete in modern web development.

What You’ll Learn

Purpose of codebase Attribute

The primary purpose of the codebase attribute is to provide a base URL for the browser to locate additional resources required by an embedded application or applet. By setting codebase, developers could ensure the browser knew where to find files referenced by the code or data attributes—for example, a .class file or supporting JAR archive on a remote server.

📝 Syntax

Add codebase to an <applet> or <object> element with a URL value:

<applet code="MyApplet.class" codebase="http://example.com/applet/" width="300" height="200">
  Your browser does not support Java applets.
</applet>

Syntax Rules

• Used on <applet> (obsolete) and <object>.
• Value is a URL—absolute or relative to the document URL.
• Works with code on applet or data / classid on object.
• Always include fallback content inside the element.
• Do not use for general page links; that is the job of <base href>.

💎 Values

The codebase attribute accepts a URL value that specifies the base URL for embedded resources. This URL can be relative or absolute. Relative URLs are resolved relative to the URL of the document containing the codebase attribute.

Absolute URL

Points to a full server path for applet or object resources:

<applet code="MyApplet.class" codebase="http://example.com/applet/">
  Fallback content
</applet>

Relative URL

Resolved against the current document’s location:

<object data="plugin.swf" codebase="assets/plugins/" type="application/x-shockwave-flash">
  <p>Plugin not available.</p>
</object>

Applicable Elements

Examples Gallery

Applet codebase URL example and dynamic JavaScript setAttribute.

<applet code="MyApplet.class" width="300" height="200" codebase="http://example.com/applet/">
  <!-- Additional applet parameters and content -->
  Your browser does not support Java applets.
</applet>

<applet id="myApplet" code="MyApplet.class" width="200" height="100">
  Your browser does not support Java applets.
</applet>

<script>
  var appletElement = document.getElementById("myApplet");
  appletElement.setAttribute("codebase", "http://example.com/dynamic/");
</script>

♿ Accessibility

• Provide fallback content — Always include accessible text inside <applet> or <object> when plugins fail.
• Avoid plugin-only features — Do not rely on applets or plugins for essential functionality.
• Use native HTML — Video, audio, canvas, and standard forms are more accessible than legacy embeds.
• Clear messaging — Tell users when embedded content is unavailable and offer an alternative.
• Keyboard access — Plugin content often lacked proper keyboard and screen reader support.

💡 Best Practices

• Resource location: Use codebase to specify the base URL for resources associated with your application or applet, ensuring the browser can locate and load them correctly.
• Accessibility of files: Ensure the specified URL is accessible and contains the necessary files and resources required by your application or applet.
• Browser compatibility: Test your application or applet across different browsers when maintaining legacy systems, as plugin support varied historically.