<br /> not valid html

Checking my markup and validating my syntax at W3C turn out <br /> and <blink></blink> as non-standards conform code. According to W3C the correct line break tag is <br>. This is confusing since you have been teaching us to use these (non-standards conform) tags. Why so?

Hi, I would like to add an observation or two if I may.
First <br /> is not the proper form for a line break in some versions of HTML. However it is in xhtml and is required for sites to be xml compliant now. it will also be required for html 5 compliance in the future. One thing every designer or developer should consider is how their work will stand up in the future.

Yes, thanks for the extensive reply.