Remember that all of these pages, sites, and apps will be accessed by users with disabilities, including blind users with screen readers, low-vision users who prefer large fonts, people who can't use a mouse, and more. You should use all of the same guidelines you'd use when writing any webpage to make sure it's maximally accessible to all users - there are thousands of resources online, just do a Google search for "Accessible HTML".
You can use the Accessibility Developer Tools extension on the Chrome Web Store as one way to help test! This extension is developed by the Chromium team and will help catch lots of common errors and hopefully make it much easier to catch accessibility errors early in the design process.
If you add WebUI tests, an accessibility audit, based on the same code as the Accessibility Developer Tools, above, is automatically run on any pages that get tests.
Here are some specific things to think about in particular for Chrome:
Focus: Make sure that your page is usable without the mouse. You should be able to press Tab to cycle through all focusable links and controls on the page. It should be very visually apparent when an element has focus, and this indication should be as consistent as possible within a page. It should be impossible for focus to get "trapped" in a single control. When the user activates a button or control that opens a dialog or pop-up, place focus inside the new pop-up. When the user closes this pop-up, restore focus to the previous place.
Hover: Don't ever use Hover as the exclusive way to activate a feature. Make sure you can achieve the same thing by focusing a control with the keyboard.
Visibility and CSS Animation: If you use opacity:0 or height:0 to make something disappear (especially for a nice animation), that item will still be part of the render tree, and screen readers will still read it! You must add some code to wait until the animation finishes and then set the hidden items to hidden, visibility:hidden, or display:none.
Color: Don't ever use color as the only distinction between one state and another, unless there are two states with very different perceived brightness levels and an accessible annotation for screen reader users. Make sure that there's a lot of contrast between text and the background color.
Support 300% zoom: Make sure everything is still usable at 300% zoom.
Prefer native elements: Don't use a clickable <div> if you can style a <a> or <button> to look like what you want. If you create a custom control out of a <div>, you must either read up on ARIA and test with at least one screen reader, or send your changes to an accessibility expert for review.
Label Images: All <img> tags must have an alt attribute with an accessible name. If an image is purely decorative or if the accessible name is already present in an adjacent element in the DOM, you can use alt="". Note that alt="" is not the same as leaving out the attribute entirely; if you leave it out, many screen readers will read the filename of the image instead. If you're using a background image that's not purely decorative, use the title attribute to provide an accessible name.
<!-- The alt text describes the meaning of the image. -->
<img src="green_dot.png" alt="Online">
<!-- Purely decorative, so use an empty alt tag. -->
<img src="separator.png" alt="">
<!-- Use a title attribute for a div with a background image. -->
<div style="background-image: url('john_doe.png')" title="John Doe">...</div>
Label Controls: Controls should have a text label. If the control is a button whose title is text already, nothing else is needed. Every other control should usually have a <label> element pointing to it or wrapping it. If that's impossible, give it a title or placeholder attribute.
<!-- Label separate from control. -->
<input id="developer_mode" type="checkbox">
<label for="developer_mode">Developer Mode</label>
<!-- Label wraps control (also valid). -->
<!-- Has placeholder. -->
<input placeholder="First name">
<!-- These need titles because the labels are implied for users who can see the screen.
(Maybe a bad example, but hopefully you get the idea.) -->
<input size="3" title="Area code">
<input size="3" title="Phone number (first 3 digits)"> -
<input size="4" title="Phone number (last 4 digits)">
Where necessary, override visible text with accessible text:
Occasionally the text that's on the screen would be confusing to a blind user with a screen reader because the text is used symboically - for example an 'X' for a close button. In that case, using the title attribute won't work because the screen reader user will still also hear the 'X'. For this case, use aria-label.
Try to avoid using aria-label when title or placeholder would work equally well.
<!-- Uses aria-label to override the 'X' text and make sure Close is spoken instead. -->
Note that when it's an interactive control (like a button, checkbox, etc.) the aria-label attribute should go on the element that has focus.