DOCTYPE stands for Document Type and is written at the beginning of the HTML file. DOCTYPE is not a tag and tells the browser the version of the HTML file. Currently in HTML5, it is simply declared as follows
What are HTML semantic elements, and why would we like to use them?
The semantic elements in HTML play an important role in clearly conveying their meaning to both the browser and developers. For example, <article> and <aside> are new semantic elements introduced in HTML5. <article> is used for independent, self-contained content, while <div> and <span> are called non-semantic elements and have no meaning for their content.
Writing semantic HTML makes it possible to convey the meaning of content to SEO, accessibility (screen reader software), developers, and others.
When you shared a web page link to SNS like Twitter, the image, title, and description did not appear. How will you fix this?
In SNS like Twitter, the Open Graph Protocol reads properties to display information about the page. If it's not displayed on the SNS, you need to check whether information such as og:title is written in the HTML meta tag.