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After a re-design of some pages in our site, we have the new re-designed pages with doctype HTML5 and all the rest pages (that are not yet redesigned) with doctype XHTML 1.0 Transitional.

Does that create any problem?

There is a similar question (Is it bad to have a mix of HTML 5 and XHTML pages within one website from SEO perspective?) but it only touches the SEO subject. I am wondering if any other kind of issue can arise from this situation.

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    Some JS/CSS might behave unexpected due to the difference. It's the sneaky ones that kill ya – Martijn May 14 '14 at 10:14
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There is clearly no problem about it. Every request returns one document that has its own independent doctype. This is not affecting any other sites.

According to https://stackoverflow.com/questions/10380346/css-styles-not-applied-properly-if-use-doctype different doctypes could define different default CSS property values of specific (meta) elements. So it's important to overview the result, before publishing any doctype changes.

I'll leave the SEO part of your question, since you already found a link that answers this. (It does not affect your SEO.)

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    Doctypes do not affect POST data in any way and, to be clear, it only affects the box model interpreted by browsers and not the definition itself but he's using the transitional doctype and this isn't an issue. And, no, this has no affect on SEO. – Rob May 14 '14 at 12:50
  • Thx for clarifying the thing with the POST data. That's why I said it "could" not it "will". Well, I removed it from my answer. – Martin Braun May 14 '14 at 12:52
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    No HTML doctype specifies default values for CSS. (Some specs have examples but nothing normative.) And as far as I know, no browser implements different default CSS for different doctypes. – DisgruntledGoat May 14 '14 at 14:09

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