Block elements, by default, start on a new line and occupy the full width of their container, pushing content before and after to new lines. In contrast, inline elements flow within the content and only take up as much width as necessary, not causing a line break. This characteristic fundamentally affects how they shape the layout and flow of content on a web page. 

In HTML, comments are written between . It's essential to know this as comments can be used to provide explanations or annotations within the code, but they won't be displayed on the web page. This is different from some other programming and scripting languages where other symbols might be used for comments. 

The required attribute in HTML ensures that the browser prompts the user to fill in the field before allowing form submission. It's a native way to enforce field completion without relying on additional scripts or manual checks. While JavaScript can achieve similar functionality, using the required attribute is more straightforward and doesn't depend on users having JavaScript enabled. 

In CSS, the display and visibility properties are commonly used to hide and show elements. Specifically for table columns, setting the display property to none will effectively hide the column, while the visibility property can hide an element but still take up space. It's important to differentiate between them as they affect the layout differently. 

To enable smooth scrolling to anchors on a webpage purely with HTML and CSS, you can use the CSS property scroll-behavior and set its value to smooth. This will ensure that when anchor links are clicked, the browser will smoothly scroll to the target section, rather than jumping to it instantly. While there are JavaScript solutions as well, this CSS approach is the simplest and most elegant way to achieve the effect without requiring any additional scripts. 

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When building a responsive table, especially when reflowing or reformatting rows and columns based on device width, cells that span multiple rows or columns (colspan and rowspan) can pose challenges. They might not adjust properly on all screen sizes, leading to visual inconsistencies or data being hard to read. In such cases, media queries or script-based adjustments might be necessary to ensure the table remains readable and usable across devices. 

While it's technically possible to store metadata within HTML comments, it's not a recommended practice. Metadata is often better placed in tags within the of an HTML document where they're specifically designed to be. Storing metadata in comments won't cause errors, but it can lead to confusion and isn't semantically correct. Moreover, search engines and other web services won't recognize or utilize metadata stored this way.