Back-end web development concerns a far different set of technical sensibilities from front-end web development. While the latter is familiar to more people because it is primarily about the HTML and CSS syntax that dictates how a website is presented to users viewing it through a web browser, the former is concerned with the internal workings of a website. A web developer that specializes in programming work for a website's server and the internal database that the website draws from is not always proficient in the layout-focused design sense that front-end developers use as part of their work.
HTML and CSS are considered front-end programming languages and tend to be seen as more basic than back-end languages like Java, Python, and PHP because a basic website only needs to be made up of front-end syntax in order to look appealing and provide static information. Nonetheless, someone who works purely as a back-end developer can be distant enough from a front-end mentality that they might find it unusually difficult to become confident that they can take a previously illustrated design for a web page's layout and efficiently recreate it in front-end languages. The relative simplicity of the front-end languages, however, should make getting used to designing web pages with them mostly a matter of repeatedly practicing with them.
If a back-end programmer would like to practice front-end design with a lot of example layouts that other front-end developers had used for their own projects, he or she should look at various websites in the vein of Uplabs, Behance, and Dribbble. All of these websites function as self-promotion outlets in which users post their own artistic creations for public display. Meanwhile, web browser programs allow users to right-click on any area of any webpage and select an "Inspect" option in order to open up a browser feature that will show the source code of the page and highlight the syntax relevant to the area that was right-clicked. Through this function, web developers can easily remind themselves how certain parts of a website are expressed through HTML. For more information click here https://www.reddit.com/r/web_design/comments/8q1551/where_can_i_find_a_large_collection_of_mockups_to/.