The correct way to add a subject to a mailto: link is by appending ?subject=YourSubject to the end of the email address. This way, when the user clicks on the link, their default email client will open a new message with the specified subject pre-filled. The use of the question mark (?) indicates the beginning of query parameters in a URL. 

Clicking on an internal document link (also known as an anchor link) navigates to a different part of the same page. This action does not create a new entry in the browser's history since you're not navigating to a new page; rather, you're moving within the current one. Therefore, the browser's history remains unaffected. 

The usemap attribute in the tag links the image with a specific image map defined elsewhere in the HTML document. It takes the name of the map (preceded by a #) as its value. This allows multiple areas within the image to be clickable and direct to different destinations. 

Comments in HTML won't be displayed on the rendered web page that users see in their browser. However, if a user decides to view the page source (often available via right-clicking on the page and selecting 'View Page Source'), they will be able to see all the HTML code, including the comments. 

The value _blank for the target attribute instructs the browser to open the linked document in a new browsing context, typically a new tab or window. Using this ensures that the current page remains open while the user views the content of the linked document, which can be beneficial for retaining the user on your page while allowing them to explore additional content. 

The correct DOCTYPE declaration for HTML5 is . It is a way to tell the browser that the page should be rendered using HTML5 specifications. 

The start attribute in an ordered list (

) specifies the start value of the first item in the list. For instance,
will begin the list numbering from 3. This can be useful for continuing list numbering across different sections or after interruptions in the content. 

The rel="noopener" and rel="noreferrer" attributes are crucial for security and performance reasons. When using target="_blank", the new page has access to the window.opener object of the original page, which could expose it to potentially malicious attacks, like reverse tabnabbing. rel="noopener" prevents the new page from accessing the window.opener object, while rel="noreferrer" goes a step further by also preventing the Referer header from being sent to the new page. 

The ^{tag is used to define superscripted text, which means the enclosed text appears half a character's height above the baseline and is typically set in a smaller font size. It's commonly used to display exponents in mathematical notations or for references in text. While there is a more comprehensive $tag\; in\; MathML\; (Mathematical\; Markup\; Language)\; for\; representing\; mathematical\; notations,\; for\; simple\; exponents\; in\; HTML,\; thetag\; is\; the\; go-to\; choice.$}