- The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the document is written in.
- Always add the <!DOCTYPE> declaration to your HTML documents, so that the browser knows what type of document to expect. In other words, each document is analysed individually.
The declaration in the form of <!DOCTYPE html> (all lower case is allowed) is the form that has been adopted for the so-called HTML5; but in fact this will be the declaration for all versions of HTML from here on. This declaration is backward compatible although if you run the code through the validator, it will come with errors/warnings if the code is not consistent with HTML5.
To answer your question, no it will make no difference.
as in addition to the comments made by Ben: Some days before I thoroughly revamped and freshened up these "declarations" by using "Find & Replace". It's possible simultaneously for multiple web pages and so it has been possible to achieve this changeover in a short period of time and low effort.
I would like to say, that this small "investment" pays back immediately.
Actually <!DOCTYPE html> is the heading I used as others have guided me. Thank you for clarifying how the DOCTYPE works. I thought it a good idea to ask before leaping.