I always use a DOCTYPE, I always use the correct tags and attributes for the DOCTYPE, and I never use non-standard tags/attributes. But here's where it gets tricky: not all of the ads I put on my website (amazon, for example) do much for the validation. For example, two ads on my site right now cause validation errors on w3.org that wouldn't be there if it wasn't for those ads. I don't want mess my website up, but I need the ads to stay all the same. In situations such as these, is it still critical for your website to pass validation tests with flying colors? I rarely see a website without some sort of third-part ad on it, so am I right in assuming that it will cause very little to no trouble?
Unless you are serving content as XHTML no. To be honest, malformed HTML is being given by a lot of developers who are complying with XHTML strict. Validation isn't the be all and end all of code, but aim to have as few errors as possible, and if you can fix any of them yourself, do.
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