How should concatenation be used with HTML according to the provided guidelines?

Concatenation in HTML is an essential technique for dynamically generating content by combining strings and variables. The guideline that suggests using double pipes on each side of the variable is correct because this syntax is often used to properly concatenate strings while ensuring that variables are interpreted correctly in the context of HTML.

Using double pipes allows for a clear separation between pure text and variable content, thereby reducing the risk of syntax errors and improving code readability. This practice helps in correctly displaying the intended concatenated output without confusion, especially when mixing static text with dynamic data pulled from variables.

In contrast, other choices do not align as effectively with established concatenation practices in HTML coding. Using single pipes or excluding quotes around variables may lead to unexpected behaviors or syntax errors. Additionally, while it might seem like using variables without quotes is a straightforward approach, it can lead to issues with variable interpretation if not carefully handled. The guideline regarding placing text strings in double quotes emphasizes the importance of clarity and formatting but does not address the concatenation of strings with variables effectively as the correct choice does.