Could you design an integrated, robust code snippet utilizing HTML, CSS and JavaScript that can be seamlessly embedded within a webpage's existing framework to generate an age verification pop-up dialogue box? This interactive pop-up should be user-friendly, intuitive, and should be programmed to appear immediately as the webpage loads, prompting users to enter their date of birth before gaining access to the site's content. Please ensure the code snippet incorporates a user-input validation feature to cross-verify the age entered by the user against the minimum age requirement set for granting access. Also, integrate a customizable message that will be displayed to users during this process. Elaborate on the detailed functionality of each component of the code snippet, and provide placeholders for the custom user message [USERMESSAGE] and the minimum required age for access [MINIMUMAGE_LIMIT].

"What are some best practices for setting up an A/B test on [WEBSITE NAME] for [SPECIFIC ELEMENT/FEATURE], including [NUMBER] variations and sample size calculations?"

"How can [WEBSITE NAME] analyze the results of an A/B test using [ANALYTICS TOOL] and identify which variation performed better for [TARGET AUDIENCE] and [BUSINESS OBJECTIVE]?"How can [WEBSITE NAME] analyze the results of an A/B test using [ANALYTICS TOOL] and identify which variation performed better for [TARGET AUDIENCE] and [BUSINESS OBJECTIVE]?"

"What are some advanced A/B testing techniques that [WEBSITE NAME] can use to test [SPECIFIC ELEMENT/FEATURE] and increase [BUSINESS METRIC] in [TIME PERIOD], such as [PERSONALIZATION/SEGMENTATION] and multivariate testing?"

"What are some advanced A/B testing techniques that [WEBSITE NAME] can use to increase [BUSINESS METRIC] in [TIME PERIOD], such as [PERSONALIZATION/SEGMENTATION] and multivariate testing?"