What, exactly, is the DOM?

Last updated: February 8, 2026Reviewed date: February 8, 2026Reading time: 3 min read

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Article Summary

The Document Object Model, or the “DOM”, is an interface to web pages. It is essentially an API to the page, allowing programs to read and manipulate the page’s content, structure, and styles. Let’s break this down. How is a web page built? How a browser goes from a source HTML document to displaying a styled and interactive page in the viewport is called the...

Educational health guideWritten for patient understanding and clinical awareness.

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Emergency safety firstUrgent warning signs are highlighted below.

Seek urgent medical care if you notice

These warning signs are general safety guidance. Local emergency numbers and clinical judgment should always come first.

Severe symptoms, breathing difficulty, fainting, confusion, or rapidly worsening illness.
New weakness, severe pain, high fever, or symptoms after a serious injury.
Any symptom that feels urgent, unusual, or unsafe for the patient.

Emergency now

Use emergency care for severe, sudden, rapidly worsening, or life-threatening symptoms.

See a doctor

Book a professional medical evaluation if symptoms persist, worsen, recur often, affect daily activities, or occur in a high-risk patient.

Learn safely

Use this article to understand possible causes, tests, treatment options, prevention, and questions to ask your clinician.

How is a web page built?

How a browser goes from a source HTML document to displaying a styled and interactive page in the viewport is called the “Critical Rendering Path”. Although this process can be broken down into several steps, these steps can be roughly grouped into two stages. The first stage involves the browser parsing the document to determine what will ultimately be rendered on the page, and the second stage involves the browser performing the render.

The result of the first stage is what is called a “render tree”. The render tree is a representation of the HTML elements that will be rendered on the page and their related styles. In order to build this tree, the browser needs two things:

The CSSOM, a representation of the styles associated with elements The DOM, a representation of the elements

How is the DOM created (and what does it look like)?

The object structure of the DOM is represented by what is called a “node tree”. It is so called because it can be thought of as a tree with a single parent stem that branches out into several child branches, each which may have leaves. In this case, the parent “stem” is the root element, the child “branches” are the nested elements, and the “leaves” are the content within the elements.

Let’s take this HTML document as an example:

The DOM is not what you see in the browser (i.e., the render tree)

What you see in the browser viewport is the render tree which, as I mentioned, is a combination of the DOM and the CSSOM. What really separates the DOM from the render tree, is that the latter only consists of what will eventually be painted on the screen.

Because the render tree is only concerned with what is rendered, it excludes elements that are visually hidden. For example, elements that have display: none styles associated to them.

The DOM is not what is in DevTools

This difference is a bit more minuscule because the DevTools element inspector provides the closest approximation to the DOM that we have in the browser. However, the DevTools inspector includes additional information that isn’t in the DOM.

The best example of this is CSS pseudo-elements. Pseudo-elements created using the ::before and ::after selectors form part of the CSSOM and render tree, but are not technically part of the DOM. This is because the DOM is built from the source HTML document alone, not including the styles applied to the element.

Despite the fact that pseudo-elements are not part of the DOM, they are in our devtools element inspector.

This is why pseudo-elements cannot be targetted by Javascript, because they are not part of the DOM.

Check your symptom safely

Hi, I am RX Symptom Navigator. I can help you understand what to read next and what warning signs need care.

Warning: Do not use this in emergencies, pregnancy, severe illness, or as a substitute for a doctor. For children or teens, use with a parent/guardian and clinician.

A rural-friendly guide: warning signs, when to see a doctor, related articles, tests to discuss, and OTC safety education.

1 Symptom 2 Severity 3 Safe guidance

First safety question

Is there chest pain, breathing trouble, fainting, confusion, severe bleeding, stroke-like weakness, severe injury, or pregnancy danger sign?

Choose quickly

All symptoms

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Start here: Write or select a symptom. The guide will show warning signs, doctor guidance, diagnostic tests to discuss, OTC safety education, and related RX articles.

Important: This tool is educational only. It cannot diagnose, treat, or replace a doctor. OTC information is not a prescription. In an emergency, contact local emergency services or go to the nearest hospital.

Safety note: This is not a prescription or diagnosis. For severe symptoms, pregnancy danger signs, children with serious illness, chest pain, breathing difficulty, stroke-like weakness, or major injury, seek urgent care.

Which doctor may help?

Start with a registered doctor or the nearest qualified health center.

What to tell the doctor

Write when the problem started and how it changed.
Bring old prescriptions, investigation reports, and current medicines.
Write allergies, pregnancy status, diabetes, kidney/liver disease, and major past illnesses.
Bring one family member if the patient is weak, elderly, confused, or a child.

Questions to ask

What is the most likely cause of my symptoms?
Which danger signs mean I should go to hospital quickly?
Which tests are necessary now, and which can wait?
How should I take medicines safely and what side effects should I watch for?
When should I come for follow-up?

Tests to discuss

Vital signs: temperature, pulse, blood pressure, oxygen saturation
Basic physical examination by a clinician
CBC, urine test, blood sugar, or imaging only when clinically needed

Avoid these mistakes

Do not use antibiotics, steroid tablets/injections, or strong painkillers without proper medical advice.
Do not hide pregnancy, kidney disease, ulcer, allergy, or blood thinner use.
Do not delay emergency care when danger signs are present.

Medicine safety and first-aid guide

This section is for patient education only. It does not replace a doctor, pharmacist, or emergency care.

Safe first steps

Rest, drink safe water, and observe symptoms carefully.
Keep a written note of symptoms, duration, temperature, medicines already taken, and allergy history.
Seek medical care quickly if symptoms are severe, worsening, or unusual for the patient.

OTC medicine safety

For mild pain or fever, ask a registered pharmacist or doctor before using common over-the-counter pain/fever medicines.
Do not combine multiple pain medicines without advice, especially if you have kidney disease, liver disease, stomach ulcer, asthma, pregnancy, or take blood thinners.
Do not give adult medicines to children unless a qualified clinician advises it.

Avoid these mistakes

Do not start antibiotics without a proper medical decision.
Do not use steroid tablets or injections casually for quick relief.
Do not delay emergency care because of home remedies.

Get urgent help if

Severe symptoms, confusion, fainting, breathing difficulty, chest pain, severe dehydration, or sudden weakness need urgent medical care.

Medicine names, dose, and timing must be decided by a qualified clinician or pharmacist after checking age, pregnancy, allergy, other diseases, and current medicines.

For rural patients and family caregivers

Patient health record and symptom diary

Write your symptoms, medicines already taken, test results, and questions before visiting a doctor. This note stays on your device unless you print or copy it.

Doctor to discuss: Doctor / qualified healthcare provider

Tests to discuss with doctor

Basic vital signs: temperature, pulse, blood pressure, oxygen level if needed
Relevant blood, urine, imaging, or specialist tests only after clinical assessment

Questions to ask

What is the most likely cause of my symptoms?
Which warning signs mean I should go to emergency care?
Which tests are really needed now?
Which medicines are safe for my age, pregnancy status, allergy, kidney/liver/stomach condition, and current medicines?

Emergency warning signs such as chest pain, severe breathing difficulty, sudden weakness, confusion, severe dehydration, major injury, or loss of bladder/bowel control need urgent medical care. Do not wait for online information.

Go to emergency care if you notice:

Severe or rapidly worsening symptoms
Breathing difficulty, chest pain, fainting, confusion, severe weakness, major injury, or severe dehydration

Doctor / service to discuss: Qualified healthcare provider; specialist depends on symptoms and examination.

Step 1
Check danger signs first
If danger signs are present, seek emergency care and do not wait for online information.
Step 2
Record the symptom story
Write when symptoms started, severity, medicines already taken, allergies, pregnancy status, and test results.
Step 3
Visit a qualified clinician
A doctor, nurse, or qualified healthcare provider can examine you and decide which tests or treatment are needed.
Step 4
Do only useful tests
Do tests after clinical assessment. Avoid unnecessary tests, random antibiotics, or repeated medicines without diagnosis.
Step 5
Follow up and return early if worse
If symptoms worsen, new warning signs appear, or treatment is not helping, return for review quickly.

Rural patient practical tips

Take a written symptom diary and all previous prescriptions/test reports.
Do not hide medicines already taken, even herbal or over-the-counter medicines.
Ask which warning signs mean urgent referral to hospital.

This roadmap is for education. A real diagnosis and treatment plan requires history, examination, and clinical judgment.

RX Patient Help

Ask a health question safely

Write your symptom story. A health professional or site editor can review it before any answer is prepared. This box is not for emergency care.

Emergency first: Severe chest pain, breathing trouble, unconsciousness, stroke signs, severe injury, heavy bleeding, or rapidly worsening symptoms need urgent local medical care now.

Frequently Asked Questions

How is a web page built?

How is the DOM created (and what does it look like)?

The DOM is an object-based representation of the source HTML document. It has some differences, as we will see below, but it is essentially an attempt to convert the structure and content of the HTML document into an object model that can be used by various programs. The object structure of the DOM is represented by what is called a “node tree”. It is so called because it can be thought of as a tree with a single parent stem…

References

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What, exactly, is the DOM?

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