使用 KaTeX 渲染一些页面上的公式：

katex-any-where.js

// ==UserScript==
// @name         Katex Here
// @namespace    https://js.zombie110year.top
// @version      0.1
// @description  当页面加载完成后，手动选择是否用 KaTeX 渲染网页上的一些公式
// @author       zombie110year
// @grant        none
// @run-at       context-menu
// @include      *
// @require      https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/katex.min.js#sha384=y23I5Q6l+B6vatafAwxRu/0oK/79VlbSz7Q9aiSZUvyWYIYsd+qj+o24G5ZU2zJz
// @require      https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/contrib/auto-render.min.js#sha384=kWPLUVMOks5AQFrykwIup5lo0m3iMkkHrD0uJ4H5cjeGihAutqP0yW0J6dpFiVkI
// ==/UserScript==
(() => {
    const KaTeXOPTION = {
        delimiters: [
            { left: "$$", right: "$$", display: true },
            { left: "\\(", right: "\\)", display: false },
            { left: "\\[", right: "\\]", display: true },
            { left: "$", right: "$", display: false },
        ],
        throwOnError: false,
    };

    async function main() {
        loadCSS().catch(err => console.error(err));
        katexHere(document.body);
    }

    /**
     * 复刻 KaTeX 官网上的例子，添加 HTML head 元素
     */
    async function loadCSS() {
        const url = "https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/katex.min.css";
        const sha384 = "zB1R0rpPzHqg7Kpt0Aljp8JPLqbXI3bhnPWROx27a9N0Ll6ZP/+DiW/UqRcLbRjq";
        let el_style = document.createElement("link");
        el_style.setAttribute("rel", "stylesheet");
        el_style.setAttribute("href", url);
        el_style.setAttribute("integrity", `sha384-${sha384}`);
        el_style.setAttribute("crossorigin", "anonymous");
        document.head.append(el_style);
        console.debug("style loaded");
    }

    async function katexHere(el) {
        renderMathInElement(el, KaTeXOPTION);
    }
    main();
})();