Final answer:
The h1 elements within the .splash-section class will have a computed font-size of 24 pixels based on the provided CSS rules.
Step-by-step explanation:
To determine the size of h1 elements within the .splash-section, you need to calculate the resulting font-size based on the given CSS rules. The base font-size for the HTML document is set to 12 pixels. The .splash-section selector increases the font-size for its children to 16 pixels. Finally, the .splash-section h1 selector specifies a font-size of 1.5em. Since 'em' is a relative unit, the final size of the h1 element is 1.5 times the font-size of its parent .splash-section, which has been set to 16 pixels.
Therefore, to calculate the size of h1 elements:
16px (parent font-size) * 1.5 (h1 font-size in em) = 24 pixels.
This means that each h1 element within the .splash-section will be styled to have a font-size of 24 pixels.