What are the pros and cons of adopting HTML 5 now for a site redesign?

@maci 

Pros of adopting HTML5:

1. Improved semantic markup: HTML5 provides new semantic tags, such as <header>, <footer>, and <article>, to improve the structure and meaning of a website's content.
2. Better multimedia support: HTML5 has built-in support for audio and video playback without the need for third-party plugins, like Flash.
3. Cross-device compatibility: HTML5 is designed to work across multiple devices and platforms, making it easier to build websites that work on desktops, tablets, and smartphones.
4. Improved form controls: HTML5 provides new form controls and validation features, making it easier to create forms that are user-friendly and accessible.

Cons of adopting HTML5:

1. Browser compatibility: While HTML5 is widely supported, there may still be some older browsers that do not fully support all of its features.
2. Limited backwards compatibility: Some older browsers may not be able to display some of the new HTML5 elements and features, meaning that websites may need to be designed to degrade gracefully in these older browsers.
3. Steep learning curve: The new semantic elements and APIs in HTML5 may take some time to learn, especially for developers who are familiar with older versions of HTML.
4. Dependence on JavaScript: Some of the more advanced features in HTML5, such as drag and drop and local storage, require JavaScript, meaning that these features may not be available in older browsers or for users who have disabled JavaScript.

@maci 

Improved performance: HTML5 introduces new features, such as local storage and web workers, which can improve the performance and responsiveness of websites.

Enhanced mobile support: HTML5 includes features like geolocation and touch events, making it easier to create mobile-friendly websites and web applications.

Improved SEO: Using HTML5's semantic markup and structured data elements can help search engines better understand and index website content, potentially improving search engine ranking.

Rich media and animation capabilities: HTML5 enables the use of canvas and SVG (Scalable Vector Graphics), allowing for the creation of interactive and visually appealing animations and graphics without the need for external plugins.

Accessibility improvements: HTML5 introduces new elements, attributes, and ARIA (Accessible Rich Internet Applications) roles, improving the accessibility and usability of websites for people with disabilities.

Cons of adopting HTML5: Lack of support in older browsers: Some older browsers may not fully support all HTML5 features, requiring developers to provide fallbacks or alternative solutions.

Development time and cost: Migrating an existing website or building a new one with HTML5 may require additional time and resources for training, testing, and debugging.

Compatibility issues: Different browsers may interpret HTML5 features differently, leading to inconsistencies in rendering and functionality, which may require additional effort to address.

Security concerns: With the introduction of new APIs and features, there may be potential security risks that need to be considered and addressed during development.

Backward compatibility: Older devices and browsers that do not support HTML5 may experience degraded functionality or have limited access to certain website features, potentially affecting user experience.