Selecting an element that has an ID is the easiest way to know you are selecting the exact item because every ID on a page has to be unique. You can give every element on the page a class ID if you want to but no two elements can have the same ID name. CSS IDs are identified in CSS with the pound sound (aka "hashtag") #.
When you think of assigning an ID to an element think of it being like a fingerprint padlock. Once programmed there is only ONE fingerprint that will open the lock. When you have a class ID on a page, you can only have one. You could have many different padlocks that could be assigned to different people with different fingerprints, but only the fingerprint programmed for each specific lock will open it.
If you can make a habit of assigning a class ID to every element on the page (each one is unique - no 2 the same) then you will find it easy to style your elements.
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