Valid special characters depend on what context they are being used in. However, in general, special characters are considered those that are not alphanumeric or punctuation marks.
For example, in programming languages, special characters include symbols such as %, &, *, $, #, @, and !, among others. These special characters are used in different ways, depending on the specific programming language, as operators or to indicate certain functions.
In passwords, valid special characters are those that increase the strength of the password, such as !, @, #, $, and %, among others. These special characters can be used alongside uppercase and lowercase letters, numbers, and punctuation marks.
In URLs, valid special characters include those that are used to encode certain characters and ensure that the URL is properly formatted and can be accessed by different devices. These special characters include %, +, and - among others.
In general, it is important to use special characters appropriately and according to the specific context in which they are being used.