Only this line here is in the tag (so far so good but no links nor mention to the source)

In a good example of blockquote everything in a block - you could say a whole paragraph or a long text - must be inside the <blockquote> tag. It indents the block and indicates that an external text was cited. A good way to complement that is adding an information about the origin of the citation. This info does not appear in the HTML page, only in the code. The information will be done by an attribute called "cite" inside the blockquote opening tag (like we do with the inline style) and it's not to be confused with the element <cite> of item 2 below.

Use CSS' (margin-left: 0;) to remove the indentation from the blockquote element.

HTML Blockquote is a DOM Object. You can access a blockquote element by using getElementById(). Example:

var x = document.getElementById("myBlockquote");

The Blockquote object also supports the standard properties and events.

Good example of cite here. The HTML <cite> tag defines the title of a creative work (e.g. a book, a poem, a song, a movie, a painting, a sculpture, etc.)

Catcher in the Rye (this title is within a <cite> tag) is a novel by Jerome David Salinger.

A complete definition and usage examples of this HTML element can be found at https://developer.mozilla.org/en-US/docs/Web/HTML/Element/cite.

(Now, just for comparison purposes, check how poorer the Brazilian portuguese version of the above page is in the same web platform: https://developer.mozilla.org/pt-BR/docs/Web/HTML/Element/cite.)

Continuing from https://developer.mozilla.org/en-US/docs/Web/HTML/Element/cite:

Look at the interesting example they give:

It has a cite inside a figcaption and they are both inside a <figure> element. Note that the paragraph tag is surrounded by a blockquote element. And here's how the code stays:

Good example of q (applyed here) and here:

Use <q> for inline, short quotations. This element is intended for short quotations that don't require paragraph breaks; for long quotations use the <blockquote> element. MDN

Browsers normally insert quotation marks around the quotation, like now. And is that it?

No. This element may include global attributes. Therefore it can be combined with the <cite>, for instance.

Again, note that the value of the <cite> attribute is a URL that designates a source document or message for the information quoted. This attribute is intended to point to information explaining the context or the reference for the quote. No more than that.

I mean, no link is established. It's only an information that stays hidden from the user and works behind the scenes for SEO and other algorithm purposes.

All this said, something like this can be built with <q> and <cite> combined:

According to Mozilla's website, Firefox 1.0 was released in 2004 and became a big success.

And the code is written as follows (for "Displaying Code" in details, see next topic, but take a glimpse of this for while):

                            <p>According to Mozilla's website,
                                <q
                                cite="https://www.mozilla.org/en-US/about/history/details/">Firefox 1.0
                                was released in 2004 and became a big success.</q></p>
                        

<abbr> for Abbreviations

From w3schools:

The HTML <abbr> tag defines an abbreviation or an acronym, like HTML, CSS, Mr., Dr., ASAP, "ATM".

Marking abbreviations can give useful information to browsers, translation systems and search-engines.

Tip: Use the global title attribute to show the description for the abbreviation/acronym when you mouse over the element.

Example

The WHO was founded in 1948. (Did you think it was Roger Daltrey's rock band?)

<address> for Contact Information

From w3schools:

The HTML <address> tag defines the contact information for the author/owner of a document or an article.

The contact information can be an email address, URL, physical address, phone number, social media handle, etc.

The text in the <address> element usually renders in italic, and browsers will always add a line break before and after the <address> element.

Example:

<bdo> for Bi-Directional Override

From Tutorial Republic:

The <bdo> (short for bidirectional override) tag is used to override the current text direction. You must specify the "dir" attribute on this element. This mandatory attribute specifies the base direction of the element's text content, which overrides the inherent directionality of characters as defined in [UNICODE].

Attributes: This element contains dir attributes which are used to specify the direction of text written inside the <bdo> element. The dir attribute contains two values which are listed below:

• <bdo dir="rtl">: The text direction from right to left (reverse the text).
• <bdo dir="ltr">: The text usual direction from left to right.

The sequence "1-2-3-4-5" was reversed in this line by this set of tag & attribute as: 1-2-3-4-5.