somethingAwfulVideoHelper.user.js

// ==UserScript==
// @name               SA Video Helper
// @namespace          https://forums.somethingawful.com/
// @include            https://forums.somethingawful.com/*
// @description        Sets embedded video size caps, auto-enables controls, embeds webms
// @author             Elysiume
// @version            1.0.0
// @updateURL          https://gist.github.com/elysiumeproto/b5b3f528699f58af4b47b83005790c83/raw/somethingAwfulVideoHelper.user.js
// @downloadURL        https://gist.github.com/elysiumeproto/b5b3f528699f58af4b47b83005790c83/raw/somethingAwfulVideoHelper.user.js
// ==/UserScript==

const styleAnchorId = "styleAnchor_c909cf7a-62a0-4c45-928b-e62703e1def2";

window.addEventListener("load", function() {
    // Enable controls for other videos.
    $("video").attr("controls", "controls");

    // Pull in every <a> tag
    $('a').filter(function() {
        // Filter it down to just the ones that are a pure link to a webm (hypothetically)
        return this.text.endsWith('.webm') && this.href.endsWith('.webm');
    }).each(function() {
        activeWebm = $(this).attr("href");
        console.log('Replacing the webm link for ' + activeWebm + '.');
        // And replace them with an embedded video
        $(this).replaceWith('<video autoplay loop controls><source src="' + activeWebm + '" type="video/webm"></video>');
    });
}, false);

function addStyle(css) {
    // We need to create an anchor element that we can attach styles to. Check if it already exists to avoid creating multiples.
    const style = document.getElementById(styleAnchorId) || (function() {
        // We want a style-type element with a unique enough ID that it hopefully isn't conflicting with something.
        const style = document.createElement('style');
        style.type = 'text/css';
        style.id = styleAnchorId;
        document.head.appendChild(style);
        return style;
    })();
    const sheet = style.sheet;
    sheet.insertRule(css, (sheet.rules || sheet.cssRules || []).length);
};

addStyle("video { max-height: 750px; max-width: 100%; } ");