This example is running in WebGL2 and should work in most browsers. You can check the WebGPU examples here.

screenshot.rs:
//! An example showing how to save screenshots to disk

use bevy::prelude::*;
use bevy::render::view::screenshot::ScreenshotManager;
use bevy::window::PrimaryWindow;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, screenshot_on_spacebar)
        .run();
}

fn screenshot_on_spacebar(
    input: Res<ButtonInput<KeyCode>>,
    main_window: Query<Entity, With<PrimaryWindow>>,
    mut screenshot_manager: ResMut<ScreenshotManager>,
    mut counter: Local<u32>,
) {
    if input.just_pressed(KeyCode::Space) {
        let path = format!("./screenshot-{}.png", *counter);
        *counter += 1;
        screenshot_manager
            .save_screenshot_to_disk(main_window.single(), path)
            .unwrap();
    }
}

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // plane
    commands.spawn(PbrBundle {
        mesh: meshes.add(Plane3d::default().mesh().size(5.0, 5.0)),
        material: materials.add(Color::srgb(0.3, 0.5, 0.3)),
        ..default()
    });
    // cube
    commands.spawn(PbrBundle {
        mesh: meshes.add(Cuboid::default()),
        material: materials.add(Color::srgb(0.8, 0.7, 0.6)),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        ..default()
    });
    // light
    commands.spawn(PointLightBundle {
        point_light: PointLight {
            shadows_enabled: true,
            ..default()
        },
        transform: Transform::from_xyz(4.0, 8.0, 4.0),
        ..default()
    });
    // camera
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });

    commands.spawn(
        TextBundle::from_section(
            "Press <spacebar> to save a screenshot to disk",
            TextStyle::default(),
        )
        .with_style(Style {
            position_type: PositionType::Absolute,
            top: Val::Px(12.0),
            left: Val::Px(12.0),
            ..default()
        }),
    );
}