Unlike the website design, during the website development you are no longer actively involved. After you have agreed to the final design, development of your website is done by your supplier in HTML, PHP or javascript .
The source code of the web pages is written to be clear, and to comply to the web standards of W3C (World Wide Web Consortium), at the same time, the source code is made to be as small as possible (the smaller the page, the faster the response for the web visitor) and to be valid according to the specifications.
After development has been finished, the code of the web pages is checked against the W3C specification, validated by W3C Validator and tested on the web server. After completion of testing the website is handed over to the customer.
Once the website development begins, it is advisable to avoid design changes. That extends the delivery of web pages and the development is becoming more expensive.