How do I create sub-classes in standard <style> tags?

For example, I may want to make create a div.St开发者_运维百科yle1 block where all <img /> elements within this type of div are given certain properties.

Is this possible? If so, what is the proper syntax?

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The syntax is

 div.Style1 img { border: 3px red solid }

see also CSS basics at W3schools