HTML5 has revolutionized web development by introducing a plethora of innovative features that make it easier for developers to create dynamic and interactive websites. Among the top features, semantic elements stand out as they enhance the document structure, making it more readable for both developers and search engines. Elements like <header>, <footer>, and <article> bring meaning to the content, which is crucial for SEO. Furthermore, with the introduction of the canvas API, developers can now render graphics on the fly, allowing for a richer user experience without relying on external plugins.

Another game-changing feature in HTML5 is the video and audio support that eliminates the need for third-party media players. By using the <video> and <audio> elements, developers can easily incorporate media content into their sites, improving engagement and user retention. Additionally, the geolocation API enables developers to create location-based applications that enhance user interaction by providing relevant content based on the user's current location. In summary, mastering these top 10 HTML5 features can significantly improve a developer's toolkit and boost their web applications' performance.

Building responsive websites using HTML5 is essential in today's digital landscape, where users access the web from a variety of devices. To start, use the viewport meta tag to ensure proper scaling on different devices. A basic example looks like this: <meta name='viewport' content='width=device-width, initial-scale=1.0'>. This tag tells the browser to adjust the page's dimensions and scaling based on the device's width. Next, leverage the power of CSS3 in conjunction with HTML5 to create fluid layouts. Utilizing percentage-based width and media queries allows you to adapt your site’s design to different screen sizes seamlessly.

In addition to responsive layout techniques, consider incorporating HTML5 features like canvas and audio tags, which enhance user interaction without the need for external plugins. When building your website, structure your content semantically using elements like <header>, <footer>, <article>, and <nav>. This not only improves accessibility but also helps search engines understand the context of your content better. Finally, always test your website on different devices and screen sizes to ensure a consistent user experience; tools like Chrome's Developer Tools can simulate various devices, making it easy to spot responsiveness issues before launch.

When working with HTML5, developers often encounter several common mistakes that can hinder the performance and accessibility of their web pages. One prevalent error is the improper use of semantic elements. For example, using a <div> tag where a <header>, <footer>, or <article> tag would be more appropriate can lead to a lack of clarity for both users and search engines. To avoid this mistake, it's essential to utilize the correct semantic tags according to the content structure. This not only improves SEO but also enhances the overall user experience.

Another frequent mistake is neglecting to implement HTML5 features such as the canvas element for graphics rendering or missing out on localStorage for better data management. Additionally, improper handling of forms, such as failing to use the correct input types like email and date, can lead to issues with user input validation. To avoid these pitfalls, developers should familiarize themselves with the full range of HTML5 elements and attributes, ensuring they are leveraging modern capabilities to create efficient and user-friendly web applications.