[Tex2002ans]

Originally Posted by angharad09

I'm used to using extra indenting (rather than quote marks) to identify a long quote -- for print and PDF documents.

That's called a blockquote. (Or officially called a "block quotation".)

Like Turtle91 said, in HTML that can be created using <blockquote>:

<p>As Einstein wrote in <i>Article Title</i>:</p>

<blockquote>
<p>This is a long quote.</p>
<p>And this is some more of a really long quote.</p>
<p>And this is the end.</p>
</blockquote>

Originally Posted by angharad09

But is indenting reliable across multiple different e-readers, including older ones?

Yes, <blockquote> works in everything since the dawn of time.

Originally Posted by hobnail

My div.block is the generic/common stuff, then I have add-ons, e.g., class="block letter". [...]

Are you using <div class="block"> instead of <blockquote>?

If a specific HTML entity exists, it's best to use it so devices can treat it properly... even if the device overrides CSS (like many Android readers) or doesn't have CSS at all.

Similar reasoning why you want to use <i> or <em> instead of <span class="italics">, or <h1> instead of <div class="heading1">.

<div> + <span> are completely blank slates... where <em> and <h1> tell the device something + may enable extra underlying functionality:

• <em> might be spoken differently by Text-to-Speech.
• Headings allow screen readers to jump around documents.

Originally Posted by Jellby

Reasonable top, left and bottom margin should be enough to set the blockquote apart from the surrounding text.

Agreed.

These past few years, for similar reasoning, I've also been tempted to remove the right margin on my blockquotes to squeeze in more text. I've been doing a lot more reading on my cellphone, and some of these large blockquotes take up enormous amount of screen.

Originally Posted by Jellby

Another "trick" I sometimes use is replacing text-indent inside the blockquote with some vertical spacing between paragraphs (not a full line, something like 0.5em).

This type of spacing is also called "block indent". (Paragraphs with no indent + gap between each paragraph.)