W3C Validator offline on localhost by ajax

I use to programming Brackets text editor and I have already installed W3C validator but it working while I'm online but offine not. I try install https://validator.w3.org/docs/install.html and I running to localhost:8888 but brackets's extension connecting only via ajax (javascript). Is possible send ajax like to original W3C website?

Maintainer of the W3C HTML checker (validator) here. Yes it is possible to send an ajax request to a local instance of the current checker. To use Fetch to do it and get JSON-formatted results back:

var checkerUrl = "http://localhost:8888/?out=json"
fetch(document.location.href)
.then(function(currentDoc) { return currentDoc.text(); })
.then(function(htmlSource) {
    fetch(
        checkerUrl, {
        method: "POST",
        mode: "cors",
        body: htmlSource,
        headers: new Headers({ "Content-Type": "text/html;charset=utf-8" })
    })
    .then(function(checkerResponse) { return checkerResponse.json(); })
    .then(function(jsonOutput) {
        console.dir(jsonOutput.messages);
    })
});

That shows the basic steps to follow to deliver the request in the way the checker expects:

• send a document to the checker as the body of a POST (in this example, the current doc)
• tell the checker to format its results as JSON (out=json)
• make text/html;charset=utf-8 the media type of the POST body you send to the checker

The checker also supports multipart/form-data for giving it the HTML source to be checked, but giving it the source as a POST body instead is the preferred (and better) way for doing it.

If instead of using fetch() you want to use JQuery $.ajax(…), here’s an example:

var checkerUrl = "http://localhost:8888/?out=json"
$.get(document.location.href,
function(htmlSource)
{
    $.ajax({
        url: checkerUrl,
        type: "POST",
        crossDomain: true,
        data: htmlSource,
        contentType: "text/html;charset=utf-8",
        dataType: "json",
        success: function (jsonOutput) {
            console.dir(jsonOutput.messages);
        }
    });
});

And if instead of either fetch() or JQuery $.ajax(…) you want to use old-school XHR but it’s not clear how to handle the details in that case, let me know and I can post an example of that too.

In all cases, the .messages JSON output is an array of objects that each contain something like:

firstColumn: 1
lastColumn: 6
lastLine: 4
message: "Unclosed element “span”."
type: "error"

The documentation for the checker JSON format gives more details of the JSON the checker emits.