I'm OK with adding the .xsl file extension to XML language, but .ssi is too generic to add to HTML language, so please remove that change.

You can write a simple extension to add the .ssi file extension to HTML as follows:

Inside Brackets, use Help > Show Extensions Folder to open the extensions folder in OS file system. Then drag that folder back onto Brackets to open folder as your current project. Then open the /user folder and create a folder for your extension called something like /add-ssi.

Create a file named main.js in the /add-ssi folder that looks something like this:

/*jslint vars: true, plusplus: true, devel: true, nomen: true, regexp: true, indent: 4, maxerr: 50 */
/*global define, $, brackets, window */
define(function (require, exports, module) {
    "use strict";
    var LanguageManager = brackets.getModule("language/LanguageManager");
    var language = LanguageManager.getLanguage("html");
    language.addFileExtension("ssi");
});

Then shutdown/restart Brackets (or use Debug > Reload With Extensions) to load your extension.

Thanks @redmunds I have made the requested change.

Looks good. Thanks. Merging.