1. Project Overview
Cat.js is a lightweight, state-of-the-art engine designed to build powerful User Interfaces (UIs). It prioritizes the highest performance and efficiency, based exclusively on modern native browser APIs, primarily Web Components.

- Version: 0.1.0 (as of October 1, 2024)

2. Core Concepts and Principles
- Utilizes modern native browser APIs, especially Web Components
- Focuses on maximum performance and efficiency
- Promotes a lightweight approach to UI development
- Emphasizes simplicity and ease of use

3. Technical Details

Key Features
- Client-Side Compiler (CSC) before loading the DOM
- Attachment through a script:src tag in the head of HTML documents
- Written in vanilla, low-level JavaScript optimized for browser runtime
- Beautiful and easy-to-use syntax
- Efficient project tree structure

Implementation Details
- No Virtual DOM or similar abstractions
- Avoids use of bundlers, Webpack, polyfills, etc.
- Discourages writing HTML or CSS code within JavaScript files
- Eliminates render methods for components and logic that trigger unnecessary reflows and repainting

4. Code

```

// cat.js

// Immediately Invoked Function Expression (IIFE) to encapsulate the engine
(function() {
  'use strict';

  /**
   * Internal module for tracking loaded files.
   */
  const Registry = (function() {
    const loadedFiles = new Set();

    return {
      isFileLoaded: (path) => loadedFiles.has(path),
      markFileAsLoaded: (path) => loadedFiles.add(path),
    };
  })();

  /**
   * Compiler module for generating self-contained component classes.
   */
  const Compiler = (function() {
    /**
     * Compiles the component's template and script into a component class definition.
     * @param {string} name - The name of the component (e.g., 'my-component').
     * @param {string} templateContent - The inner HTML of the component's <template>.
     * @param {string} scriptContent - The content of the component's <script>.
     * @returns {Function} - A function that defines and registers the component.
     */
    function compileComponent(name, templateContent, scriptContent) {
      return function() {
        class ComponentClass extends HTMLElement {
          constructor() {
            super();

            const shadowRoot = this.attachShadow({ mode: 'open' });

            // Parse the template content and clone it
            const template = document.createElement('template');
            template.innerHTML = templateContent;
            shadowRoot.appendChild(template.content.cloneNode(true));

            // Process the template content to find bindings
            this._bindings = [];

            const propertyNames = new Set();

            const processNode = (node) => {
              if (node.nodeType === Node.TEXT_NODE) {
                const originalText = node.textContent;
                const regex = /\{(\w+)\}/g;
                let match;
                const matches = [];
                while ((match = regex.exec(originalText)) !== null) {
                  const varName = match[1];
                  propertyNames.add(varName);
                  matches.push({ varName, index: match.index, length: match[0].length });
                }
                if (matches.length > 0) {
                  this._bindings.push({
                    type: 'text',
                    node,
                    originalText,
                    matches,
                  });
                }
              } else if (node.nodeType === Node.ELEMENT_NODE) {
                // Process attributes
                for (let attr of Array.from(node.attributes)) {
                  const originalValue = attr.value;
                  const regex = /\{(\w+)\}/g;
                  let match;
                  const matches = [];
                  while ((match = regex.exec(originalValue)) !== null) {
                    const varName = match[1];
                    propertyNames.add(varName);
                    matches.push({ varName, index: match.index, length: match[0].length });
                  }
                  if (matches.length > 0) {
                    this._bindings.push({
                      type: 'attribute',
                      node,
                      attributeName: attr.name,
                      originalValue,
                      matches,
                    });
                  }
                }
                // Process child nodes
                node.childNodes.forEach(childNode => {
                  processNode(childNode);
                });
              }
            };

            // Start processing from shadowRoot's child nodes
            shadowRoot.childNodes.forEach(node => {
              processNode(node);
            });

            // Define reactive properties with getters and setters
            propertyNames.forEach(varName => {
              let value = '';
              Object.defineProperty(this, varName, {
                get() {
                  return value;
                },
                set(newValue) {
                  value = newValue;
                  this._updateBindings(varName);
                },
              });
            });

            // Execute user script in the context of 'this'
            if (scriptContent.trim()) {
              (function(){
                'use strict';
                eval(scriptContent);
              }).call(this);
            }
          }

          _updateBindings(varName) {
            // Update bindings that are dependent on varName
            this._bindings.forEach(binding => {
              let shouldUpdate = false;
              binding.matches.forEach(match => {
                if (match.varName === varName) {
                  shouldUpdate = true;
                }
              });

              if (shouldUpdate) {
                if (binding.type === 'text') {
                  // Recompute textContent
                  let text = binding.originalText;
                  binding.matches.forEach(match => {
                    const val = this[match.varName];
                    text = text.replace('{' + match.varName + '}', val);
                  });
                  binding.node.textContent = text;
                } else if (binding.type === 'attribute') {
                  // Recompute attribute value
                  let value = binding.originalValue;
                  binding.matches.forEach(match => {
                    const val = this[match.varName];
                    value = value.replace('{' + match.varName + '}', val);
                  });
                  binding.node.setAttribute(binding.attributeName, value);
                }
              }
            });
          }

          connectedCallback() {
            // Component is connected to the DOM
            // Any additional initialization can be done here
          }

          disconnectedCallback() {
            // Clean up if necessary
          }
        }

        // Define the custom element
        customElements.define(name, ComponentClass);
      };
    }

    return {
      compileComponent,
    };
  })();

  /**
   * Internal module for loading and parsing HTML files.
   */
  const Loader = (function() {
    /**
     * Loads an HTML file from the given path.
     * @param {string} path - The path to the HTML file (without .html extension).
     * @returns {Promise<string>} - A promise that resolves to the content of the HTML file.
     */
    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;
      }
    }

    /**
     * Parses an HTML string into a Document object.
     * @param {string} htmlString - The HTML string to parse.
     * @returns {Document} - The parsed Document object.
     */
    function parseHTML(htmlString) {
      const parser = new DOMParser();
      const doc = parser.parseFromString(htmlString, 'text/html');
      return doc;
    }

    return {
      loadFile,
      parseHTML,
    };
  })();

  /**
   * Global load function.
   * @param {string} path - The path to the HTML file (without .html extension).
   * @returns {Promise<void>}
   */
  window.load = async function(path) {
    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);

      // Extract the <template> with the 'name' attribute
      const template = doc.querySelector('template[name]');
      if (!template) {
        console.error(`No <template name="..."> found in ${path}.html`);
        return;
      }

      const componentName = template.getAttribute('name');
      const templateContent = template.innerHTML;

      // Extract the <script> content
      const script = doc.querySelector('script');
      const scriptContent = script ? script.textContent : '';

      // Use compiler to generate and define the component
      const defineComponent = Compiler.compileComponent(componentName, templateContent, scriptContent);

      // Execute the function to define the component
      defineComponent();

      console.log(`Component registered: ${componentName}`);
    } catch (error) {
      console.error(`Error loading component from ${path}: ${error.message}`);
    }
  };
})();

```

Sample component:

```
<!-- app/comp/counter.html -->
<template name="my-counter">
    <style>
        .on {
            background-color: blue;
        }
        .off {
            background-color: aliceblue;
        }
        .btn {
            padding: 20px;
        }
    </style>

    <button class="{status} btn">Count: {count}</button>
</template>

<script>
    this.count = 0;
    this.status = 'on';
    
    const button = this.shadowRoot.querySelector('button');
    const span = button.querySelector('span');

    const component = this; // To maintain correct context inside functions

    function updateCount() {
        component.count++;

        component.status = component.status === 'on' ? 'off' : 'on';
        // The reactive system will automatically update the DOM elements where {count} and {status} are used
    }

    button.addEventListener('click', updateCount);
</script>
```

Your first task is analyze our current compiler, and determine which optimizations lack to outperform Svelte compiled components. Don't focus on the compiler, focus on the compiled component, that needs to be highly efficient, with 0 dependencies. Cat.js compiled components must be powerful, outperforming svelte components.

Invest all the time needed designing this optimizations, consider also edge cases and managing multiple instance of the same component. After you achieve to an architecture that outperform svelte compiled components, then output the cat.js with the implementation.

It's not about copying svelte method, it's about understanding how the browser runtime works and memory management, and from there build something that fits them perfectly.