From the lead page ofWebsite
Design & Maintenance:
The part visitors do not 'see' - the source code - can dramatically impact updating and search engine optimization (SEO) as well as other aspects of a website's viability.
HTML (HyperText Markup Language) contains the basic building blocks for websites and web pages.
Web browsers use HTML to display text, images and other content.
HTML can embed scripts in languages such as JavaScript which affect the behavior of web pages. Web browsers can also refer to CSS (Cascading Style Sheets) to define the appearance and layout of text and other material.
CSS enables the separation of document content from document presentation, including colors and fonts. For example, minor edits in a separate CSS file could change the font type, size & color for an entire website if all of the web pages refer to that CSS file to define the presentation of text.
This separation provides flexibility in the presentation; effective use of CSS reduces complexity and repetition allowing for site wide presentation changes to be executed quickly.
JavaScript allows web browsers to provide enhanced user interfaces and dynamic elements. Perhaps, on the home page, you experience
-Viewing Mode- (Day / Night) and -'Click & Drag' Arrows-. These are JavaScript interactive features. The Music Notation & Performance side of the home page (Common Clefs) allow for pop-up information with mouseover, also a JavaScript interactive feature.
PHP (Hypertext Preprocessor) scripting language usually runs on a web server to provide interactive features generally not provided by basic HTML and JavaScript. Perhaps the most popular application of PHP is for contact forms.
HTML, CSS, JavaScript & PHP are important elements, however, there are many more details concerning source code construction - not practical to list them all on this website.
HTML can embed scripts in languages such as JavaScript which affect the behavior of web pages. Web browsers can also refer to CSS (Cascading Style Sheets) to define the appearance and layout of text and other material. 
JavaScript interactive features. The Music Notation & Performance side of the