What a developer is asking for, and why
When a web page looks wrong for one person and fine for everyone else, the cause is nearly always something about what that person is viewing it on. These are the details that matter.
Browser and version
Browsers do not all draw a page the same way, and an old version can lack something a new one has. The name alone is not enough: two versions of Safari three years apart behave differently. The version tells the developer which browser to open to see what you see.
Operating system and device
The same browser differs between a phone and a laptop, and nearly every browser on an iPhone or an iPad is built on Safari’s engine, whatever it is called. Fonts, scrollbars and form controls come from the operating system too, so a page can look different on Windows and on a Mac in the same browser.
Screen resolution and viewport
The screen resolution is the size of the whole display. The viewport is the part of the browser window that shows the page, without the tabs, the address bar and the toolbars, and it is the one that matters most. A website’s layout changes at set viewport widths, called breakpoints: a menu might fold away below 900 pixels. A browser window that is not maximised, or one with a side panel open, can give a large screen the layout meant for a tablet. Resize this window and the viewport figure above changes while the screen resolution stays as it is.
Pixel ratio
Most screens now pack more than one physical pixel into each pixel a web page measures in. A pixel ratio of 2 means each is drawn two pixels across and two down, which is what Apple calls a Retina display. It explains why a logo can look sharp on one screen and soft on another: on a ratio of 2 an image needs twice the pixels each way to stay crisp. Zooming the page changes the figure, so set the zoom back to 100% for a true reading.
The user agent
The user agent is one line of text that names the browser and the system it runs on. It is hard to read, and browsers now keep parts of it vague on purpose, to make people harder to track: browsers on a Mac all claim the same old version of macOS, for example. That is why this page says so when it could not tell, and why it shows the whole line for a developer to read.