Server Push

Animation and live webcams before JavaScript could do either: the server just kept sending replacements.

In 2026: Partly. multipart/x-mixed-replace let the server hold the connection and push new documents or images down it, replacing the old in place. The document flavour is dead, Chrome removed it in 2013, but the image flavour is motion JPEG, and cheap IP cameras still stream it into IMG tags today. Server-Sent Events and WebSocket are its respectable descendants. A static demo cannot hold a connection open, so what is below is the wire format itself.

Where it came from: Netscape's An Exploration of Dynamic Documents, Navigator 1.1, 1995. Chrome dropped the document form in 2013; EventSource carries the idea now.
Netscape, 1995 ↗Wikipedia ↗

<pre style="font-size:12px">
HTTP/1.0 200 OK
Content-type: multipart/x-mixed-replace;boundary=NextPart

--NextPart
Content-type: text/html

&lt;h1&gt;It is 14:01&lt;/h1&gt;
--NextPart
Content-type: text/html

&lt;h1&gt;It is 14:02&lt;/h1&gt;
--NextPart--
</pre>

<p style="font:13px Verdana">Each boundary replaced the whole page in place, on
one connection that never closed. Point the same trick at a stream of JPEGs and
you have every webcam page of 1996, and most cheap IP cameras still.</p>