htmx Head Tag Support Extension

The head-support extension adds support for head tags in responses to htmx requests.

htmx began as a library focused on partial replacement of HTML within the body tag. As such, merging additional information such as the head tag was not a focus of the library. (This is in contrast with, for example, TurboLinks, which was focused on merging entire web pages retrieved via AJAX into the browser.)

The hx-boost attribute moved htmx closer to this world of full HTML-document support & support for extracting the title tag out of head elements was eventually added, but full head tag support has never been a feature of the library. This extension addresses that shortcoming.

Install

<script src="https://unpkg.com/htmx-ext-head-support@2.0.1/head-support.js"></script>

Usage

<body hx-ext="head-support">
...
</body>

With this installed, all responses that htmx receives that contain a head tag in them (even if they are not complete HTML documents with a root <html> element) will be processed.

How the head tag is handled depends on the type of htmx request.

If the htmx request is from a boosted element, then the following merge algorithm is used:

Elements that exist in the current head as exact textual matches will be left in place
Elements that do not exist in the current head will be added at the end of the head tag
Elements that exist in the current head, but not in the new head will be removed from the head

If the htmx request is from a non-boosted element, then all content will be appended to the existing head element.

If you wish to override this behavior in either case, you can place the hx-head attribute on the new <head> tag, with either of the following two values:

merge - follow the merging algorithm outlined above
append - append the elements to the existing head

Controlling Merge Behavior

Beyond this, you may also control merging behavior of individual elements with the following attributes:

If you place hx-head="re-eval" on a head element, it will be re-added (removed and appended) to the head tag on every request, even if it already exists. This can be useful to execute a script on every htmx request, for example.
If you place hx-preserve="true" on an element, it will never be removed from the head

Example

As an example, consider the following head tag in an existing document:

<head>
<link rel="stylesheet" href="https://the.missing.style">
<link rel="stylesheet" href="/css/site1.css">
<script src="/js/script1.js"></script>
<script src="/js/script2.js"></script>
</head>

If htmx receives a request containing this new head tag:

<head>
<link rel="stylesheet" href="https://the.missing.style">
<link rel="stylesheet" href="/css/site2.css">
<script src="/js/script2.js"></script>
<script src="/js/script3.js"></script>
</head>

Then the following operations will occur:

<link rel="stylesheet" href="https://the.missing.style"> will be left alone
<link rel="stylesheet" href="/css/site1.css"> will be removed from the head
<link rel="stylesheet" href="/css/site2.css"> will be added to the head
<script src="/js/script1.js"></script> will be removed from the head
<script src="/js/script2.js"></script> will be left alone
<script src="/js/script3.js"></script> will be added to the head

The final head element will look like this:

<head>
<link rel="stylesheet" href="https://the.missing.style">
<script src="/js/script2.js"></script>
<link rel="stylesheet" href="/css/site2.css">
<script src="/js/script3.js"></script>
</head>

Events

This extension triggers the following events:

htmx:removingHeadElement - triggered when a head element is about to be removed, with the element being removed available in event.detail.headElement. If preventDefault() is invoked on the event, the element will not be removed.
htmx:addingHeadElement - triggered when a head element is about to be added, with the element being added available in event.detail.headElement. If preventDefault() is invoked on the event, the element will not be added.
htmx:afterHeadMerge - triggered after a head tag merge has occurred, with the following values available in the event detail:
added - added head elements
kept - kept head elements
removed - removed head elements
htmx:beforeHeadMerge - triggered before a head merge occurs