: What is missing in my SEO checklist for new websites? So I have a question about what can be done for SEO-purposes purely with coding (meaning this question is not about content strategies,

So I have a question about what can be done for SEO-purposes purely with coding (meaning this question is not about content strategies, keyword research, linkbuilding and stuff like that, but only about building crawlable semantic mark-up with all the necessary meta-tags). When I'm building a website, I make sure to do anything I can to make the site SEO-optimized and also ready for sharing to Facebook and other Social Media. My checklist looks like that:

Elements in <head>

<title> (title of the page)
<meta name="description"> (relevant for SERP)
<meta name="author">
Important open Graph meta-tags (so that sharing a page will look good on Facebook and other Social Media, meaning there will be a preview picture, a custom title and description text)

og:author
og:description
og:image
og:type
og:url

All of the above should be editable on a per-site basis from the backend.

Structure of the <body>

<h1> (page's topic)
<h2> - <h4> (section's topic's)
<nav> for the menu
<article> for the page's main content
<aside> for sidebars
<section> for content sections
other semantic block-tags (<p>, <blockquote>, ...)
avoid <div>s as much as possible
Of the above, there should be at least one <h1> and one <article>

So I have a couple of questions about this list:

You may have noticed that the <meta name="keyword">-element didn't make the list, because Google doesn't use it according to their website. Does it still have any relevance? Should I include it and if so, what should it contain?
A couple of the 'standard' meta-tags have a similar function as the OpenGraph meta tags (e.g. <meta name="author> & <meta property="og:author">; <meta name="description"> & <meta property="og:description">. In these cases, should I use both in conjunction? Or only one of them (and if so, which one)?
How important are og:type and og:url and how should they be used? Are there other important OpenGraph tags that I didn't mention?
How important are Twitter Cards? Should I use them all the time? Or only for specific purposes? And if so, could you give an example of a good usage?
What else should I include in the <head>?
Is my general rule where each individual page should at least include one <h1>-heading and one <article> (and other semantic HTML5-elements depending on the requirements of the page) correct? What else should be included in the <body>?
(Maybe slightly off-topic: How important is a XML-sitemap? I haven't really worked with that before, can you recommend any good tutorials on how to build a good sitemap?)

I'm aware that the coding is only one half of good SEO, the other one being creating good content including good keywords (that cater to 'long tail' searches) as well as linkbuilding. However, this question is purely about what can be done regarding SEO coding-wise.