seo – How bad is it to use elements for HTML icons?

When I go to the W3C validation service, to test if you complain about using a Element, with this example:




    Title
    


    


The error I get from him says the following:

Error: the element icon is not allowed as a child of the element's body in this
context. (Deleting more errors from this subtree.)

From line 8, column 2; to line 8, column 7

Content model for the body of the element: flow content.

The part where he says "not allowed as a son", he suggested that the The element as such may actually exist, but it is destined for something else, and therefore does not structurally fit there. But when I go to the HTML MDN element reference, there's listed item

However, using it works (I can use it to show a Fontawesome icon) and see how HTML5 suggests that we can create our own elements (and Angularjs advocates who use arbitrary element names for directives), would it really be bad to use? practice?

Would it be bad for SEO? Would the readers complain? Would the analyzers complain? Would not it be future proof?