Code to determine whether the input box can be empty through the value of the drop-down box_Form special effects

There are two files a.html and b.html, a.html is the code below, b.html is just for a jump page when the verification is passed, no code is written

because a.html is used It is a relative path, so for the program to run normally, the b.html file must be placed in the same directory as a.html.

Copy code The code is as follows:

< ;meta name="Keywords" content="">