farinspace

HiFi is a head injection and foot injection plugin (or head include, foot include, head insert, foot insert, what ever you want to call it). It basically allows you to inject <code><script></code>, <code><style></code>, <code><meta></code> and any other code you want into the head and foot areas of your posts and pages. The code injected is page-specific, this means that only the pages you want code inserted into will be affected.
This plugin is most useful when you have specific functionality that needs to be added on a per-post/page basis.

HiFi (Head Injection, Foot Injection)

HiFi is a head and foot injection plugin. It allows you to inject code into the head and foot areas of your posts and pages on a per-page basis.

stanton119

WordPress HTML allows you to add custom HTML to both the post/page BODY and the HEAD tags from the page editor.
When copying HTML into the WordPress editor it adds spurious tags which break various elements and corrupt the HTML. By saving the HTML in the custom fields dialogue the exact HTML will be output to your post/page.
Also if you have a single page which requires an extra javascript library or style sheet you normally have to add it through your themes php files. Updating the theme files can be a pain; we normally have to set up an exception for that individual page and load it there. As this information is not available when we are editing the actual pages in the WordPress editor it is quite obstructive.
With WordPress HTML, we can add the library or stylesheet directly to the post/page head without the need to change the theme files. And importantly, all from within the WordPress editor for that page.
Example libraries: 
jQuery, ChartJS, D3js, Highcharts, Prototype
More details: <a href="http://www.richard-stanton.com/wordpress/wordpress-html/" rel="nofollow ugc">WordPress HTML</a>
Code: <a href="https://github.com/stanton119/WordPress-HTML-Plugin" rel="nofollow ugc">GitHub</a>

WordPress HTML

Add raw HTML to any post/page, without the Wordpress editor breaking it.

HiFi (Head Injection, Foot Injection) vs WordPress HTML