[1] The term has been used by different technical writers to emphasize different aspects of front-end web development.
For some writers, the term has been understood more generally to refer to separation of functionality (the "behavior layer") from a web page's structure/content and presentation,[2] while other writers have used the term more precisely to refer to the use of progressive enhancement to support user agents that lack certain JavaScript functionality and users that have disabled JavaScript.
[4][5] A typical client-side dynamic web page can be conceived as consisting of four parts: the marked-up content (HTML), the style sheet (CSS), client-side JavaScript, and embedded objects such as images.
[7] Langridge later expanded upon this thought and emphasized that the core meaning of "unobtrusive" is that "if a given Web browser doesn't support the DHTML features you're using, that absence should affect the user experience as little as possible".
[1] In other words, for Langridge, "unobtrusive" principally refers to users' experience of the absence of JavaScript features in a given situation.