What do you need to learn about web front-end?

If line-height is not specified in css, what is the default value? Is it equal to font-size? Sometimes it feels like manually setting it to be the same as font-size has a different effect than leaving it unset by default.

Some Android phones open the h5 page in QQ and click Img to preview it. After previewing, long press the image to download it. Is there any way to achieve this?

Can text shadow be implemented using text-shadow? So how to display the inner shadow of text?

How to get matching results based on specified conditions in specified JSON data?