Steven Vachon

Combining HTML “minification” with cache and HTTP compression (<a href="https://wordpress.org/extend/plugins/wp-super-cache/" rel="ugc">WP Super Cache</a>, or similar) will cut down your bandwidth and ensure near-immediate content delivery while increasing your Google rankings.
This plugin will compress your HTML by shortening URLs and removing standard comments and whitespace; including new lines, carriage returns, tabs and excess spaces. Most importantly, by ignoring <code><pre></code>, <code><textarea></code>, <code><script></code> and Explorer® <code>conditional comment</code> tags, presentation will not be affected.

WP-HTML-Compression

Reduce file size by shortening URLs and safely removing all standard comments and unnecessary whitespace from an HTML document.

tufto

Before: 
http://example.com/wp-content/themes/twentyten/style.css
After: 
/wp-content/themes/twentyten/style.css
OR:
Before: 
http://example.com/wp-content/themes/twentyten/style.css
After (adding “https”): 
https://example.com/wp-content/themes/twentyten/style.css
If you are looking for a plugin that automatically converts absolute URLs to relative URLs, or for a plugin that changes URLS for src & href URL attributes check out my other plugin.
I wrote this plugin because I needed a basepath for uploaded images for my WordPress Multisite. In other words, I needed a URL to the server root without the domain. You may find this useful for the same reason, or perhaps another.
Before you copy this code and add it into your own, keep in mind that there may be future updates. Keeping the code within an activated plugin will make sure you’re notified.

i=== URLs Editor

A FUNCTION for use in shortening URL links. This plugin automatically does shorten URLs.

WP-HTML-Compression vs i=== URLs Editor