Declutter a webpage by telling which elements you want to leave visible — a JavaScript user script for Firefox's Greasemonkey, Chrome's Tampermonkey or Opera's Violetmonkey. The script uses a prompt to ask the user for a CSS selector for those of the DOM elements the user wants to leave visible.

onlyShowThese.user.js

// ==UserScript==
// @name     onlyShowThese.user.js
// @description Declutter a webpage by telling which elements you want to leave visible — a JavaScript user script for Firefox's Greasemonkey, Chrome's Tampermonkey or Opera's Violetmonkey. The script uses a prompt to ask the user for a CSS selector for those of the DOM elements the user wants to leave visible.
// @version  1
// @grant    none
// ==/UserScript==
var selector = prompt(
  `Type in a CSS selector query (documentation: https://w3c.github.io/csswg-drafts/selectors/) for the elements you want visible.

For example, to show only p elements, but not p of class ad, type in "p, p *, -p.ad".

`,
  "p, p *, -p.ad"
);
if (!selector) {
  return;
}
function onlyShowThese(selector) {
  let selectorForElementsToShow = "";
  let selectorForElementsToHide = "";

  selector.split(",").forEach((token) => {
    // Trim trailing spaces
    token = token.trim();
    // Check if the token starts with a hyphen
    if (token.startsWith("-")) {
      // Trim the hyphen and append the token to selectorForElementsToHide
      token = token.substring(1);
      if (token) {
        // Append the token to selectorForElementsToHide
        selectorForElementsToHide += `${token}, `;
      }
    } else {
      if (token) {
        // Append the token to selectorForElementsToHide
        selectorForElementsToShow += `${token}, `;
      }
    }
  });

  // Remove the trailing comma from the selectorForElementsToShow and selectorForElementsToHide
  if (
    selectorForElementsToShow &&
    selectorForElementsToShow.trim().endsWith(",")
  ) {
    selectorForElementsToShow = selectorForElementsToShow.trim().slice(0, -1);
  }
  if (
    selectorForElementsToHide &&
    selectorForElementsToHide.trim().endsWith(",")
  ) {
    selectorForElementsToHide = selectorForElementsToHide.trim().slice(0, -1);
  }
  // Select DOM elements
  console.log("Selector for the elements to show:", selectorForElementsToShow);
  if (selectorForElementsToShow) {
    let domElementsToShow = Array.from(
      document.querySelectorAll(selectorForElementsToShow)
    ).includeAncestorElements();
    // Hide what we want to be hidden
    document.querySelectorAll("*").forEach((elem) => {
      if (!domElementsToShow.includes(elem)) {
        elem.style.display = "none";
      }
    });
  }
  console.log("Selector for the elements to hide:", selectorForElementsToHide);
  if (selectorForElementsToHide) {
    let domElementsToHide = Array.from(
      document.querySelectorAll(selectorForElementsToHide)
    );
    // Hide what we want to be hidden
    document.querySelectorAll("*").forEach((elem) => {
      if (domElementsToHide.includes(elem)) {
        elem.style.display = "none";
      }
    });
  }
}
if (!Array.prototype.includeAncestorElements) {
  Array.prototype.includeAncestorElements = function () {
    let ret = this;
    this.forEach((elem) => {
      let ancestor = elem.parentNode;
      while (ancestor !== null && ancestor !== elem) {
        if (!(ancestor in ret)) {
          ret.push(ancestor);
        }
        ancestor = ancestor.parentNode;
      }
    });
    return ret;
  };
}
onlyShowThese(selector);