写一个游览器内核

```javascript // Web browser engine (WebCore) // Written in JavaScript // Define the global variables and functions needed by the engine var document; // DOM document object var window; // Window object to hold global variables var console; // Console object for logging messages to the browser console // Define a function to create a new HTML element with given tag name and attributes function createElement(tagName, attrs) { // Create an element with the given tag name var elem = document.createElement(tagName); // Set all of the provided attributes on the element for (let attr in attrs) { elem.setAttribute(attr, attrs[attr]); } return elem; } // Define a function that parses an HTML string into DOM nodes function parseHTML(htmlString) { return new DOMParser().parseFromString(htmlString, "text/html"); } // Define a function that takes an HTML string and appends it to a parent node function appendHTML(parentNode, htmlString) { let domNodes = parseHTML(htmlString).body.childNodes; for (let i=0; i < domNodes.length; i++) { parentNode.appendChild(domNodes[i]); } } // Define a function that takes an URL and fetches its content as text or HTML async function fetchURLAsTextOrHTML(url) { let response = await fetch(url); let contentType = response.headers.get("content-type"); if (contentType && contentType.includes("text/html")) { return await response.text(); } else { return await response.json(); } } // Define a main loop which will be executed on page load or when navigating between pages async function mainLoop() { let url = window.location; let htmlContent = await fetchURLAsTextOrHTML(url); document.body = createElement("body"); appendHTML(document.body, htmlContent); }