Cat.js is a lightweight, state-of-the-art engine designed to build powerful UIs, prioritizing the highest performance and efficiency, and based exclusively on modern native browser APIs, primarily Web Components.

Practices and things to avoid for Cat.js
- No Virtual DOMs or similar practices.
- No Webpack, bundlers, polyfills, etc.
- Write HTML or CSS code into a JS file, function or similar.
- Render methods for components, or logics that trigger unnecessary reflows and repainting.

Requirements for Cat.js
- Our engine it's attach through a script:src tag on the top of the head tag.
- Written on vanilla low level JavaScript that fits perfectly the browser runtime.
- Beautiful and ease Syntax and Project tree
- CSC - Client Side Compiler before loading the DOM.

So far this is the state of Cat.js

```
// cat.js

// Immediately Invoked Function Expression (IIFE) to encapsulate the engine
(function() {
    'use strict';
  
    /**
     * Internal module for tracking loaded files and components.
     */
    const Registry = (function() {
      const loadedFiles = new Set();
      const registeredComponents = new Set();
  
      return {
        isFileLoaded: (path) => loadedFiles.has(path),
        markFileAsLoaded: (path) => loadedFiles.add(path),
        isComponentRegistered: (name) => registeredComponents.has(name),
        registerComponent: (name) => registeredComponents.add(name),
      };
    })();
  
    /**
     * Internal module for handling component registration.
     */
    const ComponentManager = (function() {
      const templateCache = new Map();
  
      function registerComponent(name, templateContent) {
        if (Registry.isComponentRegistered(name)) {
          console.warn(`Component already registered: ${name}`);
          return;
        }
  
        // Cache the template content
        templateCache.set(name, templateContent);
  
        class CustomElement extends HTMLElement {
          constructor() {
            super();
            const shadow = this.attachShadow({ mode: 'open' });
            shadow.innerHTML = templateCache.get(name);
          }
        }
  
        customElements.define(name, CustomElement);
        Registry.registerComponent(name);
        console.log(`Component registered: ${name}`);
      }
  
      return {
        registerComponent,
      };
    })();
  
    /**
     * Internal module for loading and parsing HTML files.
     */
    const Loader = (function() {
      async function loadFile(path) {
        const fullPath = `${path}.html`;
  
        try {
          const response = await fetch(fullPath);
          if (!response.ok) {
            throw new Error(`Failed to load: ${fullPath} (Status: ${response.status})`);
          }
          const content = await response.text();
          return content;
        } catch (error) {
          console.error(error.message);
          throw error;
        }
      }
  
      function parseHTML(htmlString) {
        const parser = new DOMParser();
        const doc = parser.parseFromString(htmlString, 'text/html');
        return doc;
      }
  
      return {
        loadFile,
        parseHTML,
      };
    })();
  
    /**
     * Internal module for handling script execution and nested loading.
     */
    const ScriptProcessor = (function() {
      async function processScripts(doc) {
        const scripts = doc.querySelectorAll('script');
        for (const script of scripts) {
          const scriptContent = script.textContent;
  
          // Find load calls and execute them sequentially
          const loadCalls = [...scriptContent.matchAll(/load\(\s*['"]([^'"]+)['"](?:\s*,\s*['"]([^'"]+)['"])?\s*\)/g)];
          for (const match of loadCalls) {
            const componentPath = match[1];
            const targetSelector = match[2];
            await window.load(componentPath, targetSelector);
          }
  
          // Execute the script content
          if (scriptContent.trim()) {
            try {
              new Function(scriptContent)();
            } catch (error) {
              console.error(`Error executing script in ${componentPath}: ${error.message}`);
            }
          }
        }
      }
  
      return {
        processScripts,
      };
    })();
  
    /**
     * Internal module for inserting content into the DOM.
     */
    const DOMHandler = (function() {
      function insertContent(doc, targetSelector) {
        const insert = () => {
          const targetElement = document.querySelector(targetSelector);
          if (targetElement) {
            const fragment = document.createDocumentFragment();
            const bodyContent = doc.body;
            while (bodyContent.firstChild) {
              fragment.appendChild(bodyContent.firstChild);
            }
            targetElement.appendChild(fragment);
          } else {
            console.error(`Target element not found: ${targetSelector}`);
          }
        };
  
        if (document.readyState === 'loading') {
          document.addEventListener('DOMContentLoaded', insert);
        } else {
          insert();
        }
      }
  
      return {
        insertContent,
      };
    })();
  
    /**
     * Global load function.
     * @param {string} path - The path to the HTML file (without .html extension).
     * @param {string} [targetSelector] - Optional CSS selector of the target element to insert content into.
     * @returns {Promise<void>}
     */
    window.load = async function(path, targetSelector) {
      console.log(`Loading: ${path}`);
  
      if (Registry.isFileLoaded(path)) {
        console.log(`Already loaded: ${path}`);
        return;
      }
  
      try {
        const responseText = await Loader.loadFile(path);
        const doc = Loader.parseHTML(responseText);
  
        Registry.markFileAsLoaded(path);
  
        // Process components
        const templates = doc.querySelectorAll('template[name]');
        for (const template of templates) {
          const componentName = template.getAttribute('name');
          const templateContent = template.innerHTML;
          ComponentManager.registerComponent(componentName, templateContent);
        }
  
        // Process scripts and nested loads
        await ScriptProcessor.processScripts(doc);
  
        // Insert content into the target element if specified
        if (targetSelector) {
          DOMHandler.insertContent(doc, targetSelector);
        }
      } catch (error) {
        // Error has already been logged in Loader; no need to throw
      }
    };
  })();
```

Your first task involves the method `load`, which is intended solely for loading and registering a component for use. Currently, we've had to modify the engine to manage an additional parameter, `tag-selector`, to retrieve raw HTML, primarily for building views or pages. For instance, it’s utilized to set up the root view in `index.html` with `load('app', 'app')`. However, this solution lacks robustness and violates the single-purpose principle of the `load` method (loading and registering a component for use).

I don’t want the engine to handle everything as a component. Therefore, it’s your responsibility to discover the most optimal solution that embraces our principles: ease of use, clarity, efficient browser runtime, and the best performance with memory usage. You should explore multiple approaches, test them against potential edge cases, and determine the most effective one.