As of my last knowledge update in September 2021, there is no direct "CSS Object Model (CSSOM) API" in HTML. However, the CSSOM refers to the set of APIs that allow you to manipulate the CSS of a web page using JavaScript. It involves interacting with the styles of HTML elements and modifying them dynamically. The primary components and concepts related to the CSS Object Model include:

1. Document Object Model (DOM): The DOM represents the structure of an HTML or XML document and provides a way to interact with the content and structure of a web page. The DOM is the foundation for working with CSS properties and styles in the browser.

2. document.styleSheets: This property provides access to the list of stylesheets linked to a web page. You can manipulate CSS rules, styles, and properties through these stylesheets.

3. CSSStyleSheet and CSSRule: CSS stylesheets can be accessed as CSSStyleSheet objects, and the rules within these stylesheets are represented as CSSRule objects. These objects provide methods and properties for reading and modifying the styles.

4. element.style: Each HTML element has a style property that represents the inline CSS styles for that element. You can access and modify individual CSS properties using this property.

5. getComputedStyle(): This method allows you to compute the final used values of CSS properties for an element. It returns a CSSStyleDeclaration object that provides the computed styles.

Here's a basic example of how you can use the CSSOM to manipulate styles with JavaScript:

javascript// Access an element
const element = document.getElementById('myElement');

// Change its background color using inline styles
element.style.backgroundColor = 'lightblue';

// Access the computed styles
const computedStyles = getComputedStyle(element);
const color = computedStyles.color;

// Access and modify external stylesheets
const stylesheets = document.styleSheets;
for (const stylesheet of stylesheets) {
  for (const rule of stylesheet.cssRules) {
    if (rule.selectorText === '.myClass') {
      rule.style.fontWeight = 'bold';
    }
  }
}

While the CSSOM allows you to manipulate styles on the client side, it's important to consider performance and maintainability when dynamically changing styles. Additionally, keep in mind that browser support may vary for different aspects of the CSSOM, so always check the latest browser compatibility information.