Gutenberg gives you a better starting point than most page builders and no guarantees. Core blocks emit lean, mostly semantic HTML. They will still let you skip a heading level, ship an undescribed image, and pick unreadable colours.
Here is what the common blocks actually do.
What core gets right
Semantic output. A Heading block emits a real heading. A Button block emits a real button or link. A List block emits a real list. This is not true of every builder, and it matters.
The image block prompts for alt text. Right there in the sidebar, not three screens away.
Navigation has keyboard support. The core Navigation block handles submenus with the keyboard, which many custom menus do not.
Colour pairs are checked. The editor warns you when a chosen text and background combination is hard to read. It is a warning, not a block, and it does not catch everything, but it is there.
What to watch, block by block
Heading
The level dropdown is right next to the typography controls, which makes it
easy to pick a level for its size. The editor has a document outline view —
use it, and check one h1 and no skipped levels.
Image
The alt field is prompting, not enforcing. An image with no alt text still publishes.
Note the separate decorative case: if the image carries no information, an empty alt is correct and deliberate. Leaving the field blank on a meaningful image is not the same thing. See how to write alt text.
Cover
The classic contrast trap. Text over an image, with an overlay control that defaults to something too light. Set the overlay opacity high enough and measure the result.
Buttons
A Button block with no text is a button with no name. It happens when somebody uses an icon or leaves a placeholder.
Also check the width settings: a 25 per cent-width button on mobile can fall below the 24-pixel target size minimum.
Columns
Columns reorder on mobile. Check the order still makes sense, because the DOM order is what a screen reader follows, and a visual rearrangement does not change it.
Table
Core tables support a header row. Switch it on. A data table with no header row is unnavigable with a screen reader, because there is nothing to associate the cells with.
Media & Text
Fine, with one catch: the image still needs alt text, and the field is easy to miss in this block’s sidebar.
Group, Row, Stack
These emit divs. If a Group is really a page region — a navigation area, a
main content area, a complementary sidebar — it should be a landmark. You can
set the HTML element on the block in the Advanced panel: nav, main,
aside, section, footer.
Most sites never touch this, which is why main landmark missing turns up so often.
Custom HTML
Anything goes, and nothing is checked. Review these by hand.
Block themes and templates
With a block theme, the header and footer are templates, so a fault there repeats on every page. That is good news: one fix clears hundreds of findings.
When you scan, sort by what comes up most rather than by page. The top of that list is nearly always a template problem.
Patterns and reusable blocks
Worth saying because it confuses people: a pattern’s accessibility problems are real, but they are found on the pages the pattern appears in, not on the pattern itself.
Accessibility Remediation deliberately scans posts and pages only, and not template libraries or pattern collections, because nothing in those is a page a visitor can open and scanning them double-counts every fault. The reasoning is in Choosing what to check.
Questions people ask
Is the block editor itself accessible to use?
It has improved considerably and is usable with a keyboard and a screen reader, though not without friction. That is a separate question from whether the output is accessible, which is what your visitors experience.
Do block themes help?
Somewhat. Leaner markup and fewer plugins mean fewer places to go wrong. The authoring mistakes are identical.
Should I switch from Elementor to Gutenberg for accessibility?
Not on its own. Migrating a site is expensive and the mistakes travel with the author. Fix what you have first. See Elementor accessibility.
To check what your blocks actually produce, the plugin is free on WordPress.org.