When analyzing a browser's implementation, it is difficult to provide an exact assessment without examining the source code. However, it is important for the browser to ensure that any style rules added only apply to the specific element in the working Document Object Model (DOM). Classes, which are used to group related elements, may not always be suitable for this purpose.
The way classes function means that the browser cannot assume that a particular class is only assigned to a specific element like a span
, as it does not have knowledge of how the HTML is authored. This complexity is demonstrated in the example given by NichoDiaz: a class such as .helper
may not necessarily be limited to just a body > div > span
, but could also extend to a body > div > div > span
now or in the future.
Instead of assuming based on the class, the browser makes assumptions about the structure of the elements, which is more reliable. By recognizing that there is only one span
that is a child of a div
within the body
, it generates the selector body > div > span
for the element where the style rule is applied. Adding :nth-child(1)
to the selector can help control which elements the style rule affects when multiple elements are present.
If a second span
element is added after creating a style rule for the first element, the browser will generate a more specific selector like
body > div > span:nth-child(1)
. This specificity helps to avoid unintentionally applying the style rule to both elements, emphasizing the intention to highlight and style a single specific element.