How "safe" are Angular 2 custom html tags? (selectors: Custom tags vs. Custom attributes)

Question

This is a question regarding Angular 2 selectors, Custom tags vs. Custom attributes, SEO and browser rendering.

When I first started to look over Angular 2, the very first thing I did when following their quickstart, right of the bat, was to change my selector to '[my-component]' (attribute selector) instead of 'my-component' (tag selector), so I could have <div my-component></div> in my html instead of <my-component></my-component>, which isn't valid html. So I would write html according to standards. Well, at least pretty close to standards (because my-component isn't a valid html attribute, but I could live with only that html validation error)

Then, at some point in a video on youtube, someone from the angular team mentioned that we should use the tag selector, performance wise at least.

Alright I said, screw html validation... or shouldn't I?
So:

1. Say I ignore the W3C screaming about my html being completely invalid because of the <custom-tags>. I actually have another bigger and more real concern: how does this impact SEO?
   I mean don't just think client-side app, because in the real world (and for my angular 2 project as well) I also have server-side rendering, for 2 very important reasons: SEO and Fast initial rendering of the site to the user for that initial view, before the app bootstraps. You can not have a very high traffic SPA otherwise.
   Sure, google will crawl my site, regardless of the tags I use, but will it rank it the same in both scenarios: one with <custom-make-believe-tags> and the other with only standard html tags?

2. Let's talk browsers and css:

As I started to build my first SPA site in Angular 2, I was immediately faced with another concern:
Say (in a non SPA site) I have the following html markup:

<header>
    <a class="logo">
        ...
    </a>

    <div class="widgets">
        <form class="frm-quicksearch"> ... </form>
        <div class="dropdown">
            <!-- a user dropdown menu here -->
        </div>
    </div>
</header>

<div class="video-listing">
    <div class="video-item"> ... </div>
    <div class="video-item"> ... </div>
    ...
</div>

Angular 2 wise I would have the following component tree:

<header-component>
    <logo-component></logo-component>
    <widgets-component>
        <quicksearch-component></quicksearch-component>
        <dropdown-component></dropdown-component>
    </widgets-component>
</header-component>

<video-listing-component>
    <video-item-component></video-item-component>
    ...
</video-listing-component>

Now, I have 2 options. Let's just take the <video-listing-component> for example, to keep this simple... I either
A) place the entire standard html tags which I already have (<div class="video-item"></div>) within the <video-item-component> tag, and once rendered will result in this:

<video-listing-component>
    <div class="video-listing>
        <video-item-component>
            <div class="video-item>...</div>
        </video-item-component>
        ...
        ...
    </div>
</video-listing-component>

OR:

B) Only put the content of <div class="video-item"> directly into my <video-item-component> component and adding the required class (class="video-item") for styling on the component tag, resulting in something like this:

<video-listing-component class="video-listing">
    <video-item-component class="video-item"></video-item-component>
    <video-item-component class="video-item"></video-item-component>
    ...
</video-listing-component>

Either way (A or B), the browser renders everything just fine.
BUT if you take a closer look (after everything is rendered in the dom, of course), by default the custom tags don't occupy any space in the dom. They're 0px by 0px. Only their content occupies space. I don't get it how come the browser still renders everything as you would want to see it, I mean in the first case (A):
While having float: left; width: 25%; on the div class="video-item", but each of these divs being within a <video-item-component> tag, which doesn't have any styling... Isn't it just a fortunate side-effect that the browser renders everything as you'd expect? With all the <div class="video-item"> floating next to eachother, even though each of them are within another tag, the <video-item-component> which does NOT have float: left? I've tested on IE10+, Firefox, Chrome, all fine. Is it just fortunate or is there a solid explanation for this and we can safely rely for this kind of markup to be rendered as we'd expect by all (or at least most) browsers?

Second case (B):
If we use classes and styling directly on the custom tags (<video-item-component>)... again, everything shows up fine. But as far as I know, we shouldn't style custom components, right? Isn't this also just a fortunate expected outcome? Or is this fine also? I don't know, maybe I'm still living in 2009... am I?
Which of these 2 approaches (A or B) would be the recommended one? Or are both just fine?

I have no ideea!!

EDIT:
D'oh, thanks Günter Zöchbauer. Yeah, since my divs have float: left, that's why the (custom or not) tag they're wrapped in doesn't expand it's height. Seems I've forgotten how css works since I started to look over Angular 2:)
But one thing still remains:
If I set a percentage width on a block element (call it E), I would assume it takes x% of it's immediate parent. If I set float: left, I would expect floating within the immediate parent. In my A case, since the immediate parent is a custom tag with no display type and no width, I would expect for things to break somehow, but still... my E elements behave like their parent isn't the custom tag they're each wrapped in, but the next one in the dom (which is <div class="video-listing> in my case). And they occupy x% of that and they float within that. I don't expect this to be normal, I would think this is just a fortunate effect, and I'm afraid that one day, after some browser update... I'll wake up to find all my Angular 2 sites looking completely broken.

So... are both A and B an equally proper approach? Or am I doing it wrong in case A?

EDIT2:
Let's simplify things a bit. As I got part of my question answered, let's take another example of generated html (simplified a bit, with inlined css):

<footer>
    <angular-component-left>
        <div style="float: left; width: 50%;">
            DIV CONTENT
        </div>
    </angular-component-left>
    <angular-component-right>
        <div style="float: left; width: 50%;">
            DIV CONTENT
        </div>
    </angular-component-right>
</footer>

In the original, not yet implemented html (whithout <angular-component-...>, those divs should float left and each occupy 50% of the <footer>. Surprisingly, once they're wrapped in the <angular-component-...> custom tags, they do the same: occupy 50% of the footer. But this just seems like good fortune to me, dumb luck... Unintended effect.
So, is it or isn't it "dumb luck"?
Should I leave it like that, or rewrite so instead of the above code, I would have something like this:

<footer>
    <angular-component-left style="display: block; float: left; width: 50%;">
        DIV CONTENT
    </angular-component-left>
    <angular-component-right style="display: block; float: left; width: 50%;">
        DIV CONTENT
    </angular-component-right>
</footer>

Note that the inline styling is introduced here for simplicity, I would actually have a class instead which would be in an external css file included in the <head> of my document, not through style or styleUrls from my angular components.

Answer 1

The issue is your HTML validator. The - in the element name is required for elements to be treated as custom elements and it is valid HTML5. Angular doesn't require - in element names but it's good practice.

Check for example https://www.w3.org/TR/custom-elements/#registering-custom-elements (search for x-foo) or https://w3c.github.io/webcomponents/spec/custom/#custom-elements-custom-tag-example. I'm sure this dash rule is specified somewhere but wasn't able to find the spec. It is for example required in Polymer that depends on elements being proper custom elements while this doesn't matter much in Angular. The only difference as far as I know is that when you query the element, you get a HTMLUnknownElement when the - is missing in the name and a HTMLElement when it contains a -.

See also this question I asked a few years ago Why does Angular not need a dash in component name

BUT if you take a closer look, by default the custom tags don't occupy any space in the dom. They're 0px by 0px. Only their content occupies space. I just don't get it how come the browser still renders everything as you would want to see it

I'm not sure I understand this question. When Angular processes the template it adds the content dynamically. When you see the content in the browser than it's also available in the DOM and has actual dimensions.

Search engine crawlers are able to process pages that are generated by JavaScript. If this isn't enough, server-side rendered pages can provide static HTML to crawlers that contain the whole view.